CA2168710C - Ground detection circuit for a three wire power supply - Google Patents

Abstract

A three wire power supply circuit is provided comprising a circuit for monitoring the integrity of the ground connection of the power supply circuit. A circuit element is provided in the line-to-ground current path which conducts when current flows in the line-to-ground current path and is nonconductive when the ground connection is poor or absent. A microcontroller monitors the output of the circuit element and operates a buzzer or a light emitting diode to indicate when the ground connection is poor or absent.

Description

GROUND DETECTION CIRCUIT FOR A THREE WIRE POWER SUPPLY

A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of t:he patent disclosure, as it appears in the Patent and Trademark Office files or records, but otherwise reserves all rights under U.S. copyright laws whatsoever.

Field of the Invention The invention relates to three wire electrical power supply circuits for connecting a load to an alternating current (AC) power source with nieans for detecting and annunciating when the ground connection is poor or absent. The invention further relates to transducer and load control systems with means for providing switched DC vo:ltages to circuit components.

Background of the Invention A number of electri.cal power supply circuits such as wall switch units for lighting fixtures are potentially hazardous to individuals (e.g. repairmen). Devices such as wall switches are usually connected to the ground terminal. of a three wire AC
power source in order to connect the wall switch with earth ground. This reduces the likelihood that circuit component failures and deterioration of wiring insulation will expose a repairman or operator to a dangerously high potential or current relative to earth grounci.
A need exists for a circuit which notifies a repairman or other user when the ground connector is poor or absent in a power supply ci_rcuit. T:}iis notificat.ion sliould be provided each time the power supply circuit is powered on or after it is lost during normal operation of the device. Thus, before a repairman commences servicing a wa3.1 switch unit, there is assurance that the ground connection is present.

Summary of the Inventiori In accordance with an aspect of the invention, a three wire power supply circuit is provided which comprises a circuit element in the line-to-ground current path. The circuit element conducts when current flows in the line-to-ground current path and is a non-conductive when the ground connection is poor or absent.
Accordingly the present inverition in one broad aspect provides an apparatus for monitoring a ground connection in a three wire power supply circuit which selectively connects and disconnects a:Load from an alternating current power source having line, neutral and ground conductors. The ground connection monitoring apparatus comprises generating means for generating an output signal, control means connected between the line conductor and the ground conductor and connected to the generating means, the control means being configured to operate the generating means to generate the output signal when current flows between the line conductor and the ground conductor and to interrupt operation of the generating means when current does not flow between t:he line conductor and the ground conductor.
Indicating means is connected to at least one of the control means and the generating means and operable in response to the output signal to indicate at least one of the presence and the absence of current flow :i.n the ground conductor.
Another aspect of the invention provides a method of monitoring a ground connection in three wire power supply circuit which selectively connects and disconnects a load from an alternating current power source having line, neutral and ground ~conductors, the method comprising the steps of switching a circuit element in a current path comprising the line conductor and the ground conductor into a conducting state to generate an output signal when current flows through the current path, the circuit element operating in a nonconducting state and generation of the output signal being interrupted when current is not flowing through, the current path, and monitoring the circuit element for thc: output signal to determine when the circuit element is in a conducting state. An indicator is operated when the circuit element is in one of a conducting state and a nonconducting state.
More particularly, a microcontroller monitors the output of the circuit element and operates a buzzer or a light emitting diode to indicate when the ground connection is poor or absent.
Alternatively, the microcontroller can operate an audible and/or a visible indicator to indicate when the ground connection is present.

In accordance with another aspect of the invention, the microcontroller is progy'ammed to monitor the output of the circuit element and to operate the indicator each time the power supply circuit is powered on by an AC power source or after the power supply ciz=cuit is dx-opped during normal operation.

Brief Description of the Drawinas These and other features and advantages of the present invention will be more readily apprehended from the following detailed description wheri read in connection with the appended drawings, which form a part of this original disclosure and wherein:
Figs. 1A, 1B and 1C provide a schematic circuit diagram of a transducer and load control system having a power supply circuit constructed in accordance with an embodiment of the present invention; and Fig. 2 is a flow cha.rt. depicting a sequence of operations for operating the system depicted in Figs. 1A, 1B and 1C.

Detailed Description of the Preferred Embodiments Figs. 1A, 1B and 1C collectively provide a schematic circuit diagram of a transducer and load control system 8 comprising a three wire power supply circuit 10 constructed in accordance with an embodiment of the present invention. Boxes l0a and lOb, which are drawn in phantom lines in Figs. 1A and 1C, respectively, collectively illustrate the power supply circuit 10. In the illustrated embodiment, the system 8 controls the supply of power to a lighting fixture using a passive infrared motion sensor. The system 8, however, can be used with a number of different types of loads such as smoke alarms, dimmers, as well as security and temperature control systems, among others.
The power supply circuit 10 connects a load 12 such as the lighting fixture to an AC power source 14. The load 12 is connected to the neutral conductor 16 of the AC power source 14.
The power supply circuit 10 is connected to the load 12 via a load conductor 18 and is connected to the AC power source via the AC power or hot line conductor 20.
As shown in Fig. 1B, the system 8 comprises a microcontroller 22 which is preferably a microcontroller number PIC16LC73 manufactured by Microchip Technologies, Inc., Arizona.
The power supply circuit 10 comprises a switch mechanism Ki for controllably completing or interrupting the current path between the line or power conductor 20 and the return path to the AC
source, i.e. the load conductor 18, the load 12 and the neutral conductor 16. The switch mechanism Ki can be, but is not limited to, a slide switch, a press switch, a relay, a semi-conductor switch, an opto coupler, a thyristor, or any other mechanical, electromechanical or electronic device for opening and closing a circuit. The switching mechanism Ki can be controlled manually (e.g. a press button or slide switch SW1), or by an electronic control circuit which can include, but does not require, a microcontroller. For example, the switch mechanism K1 of the circuit 10 can be provided in the form of a relay which is switched to the ON position by the microcontroller 22 to provide power to the load 12 and to the OFF position to power down the load 12. The coil for relay Ki is in the relay driver circuit 32 (Fig. 1B).
In Fig. 1A, the circuit 10 uses two half-wave rectifier circuits which operate on alternate half-cycles of the input power. The diodes Di and D3 and the resisters R78 and R79 constitute a first half-wave rectifier circuit in the line-to-load current path. Diodes D2 and D4 and resisters R64, R68 and R69 represent a second half-wave rectifier circuit for the line-to-ground current path.
When the relay Ki is open (i.e. OFF) and input power is at a positive polarity with respect to ground, diodes D1 and D3 conduct. Current flows through the ferrite bead FB1, diode Dl, resistor R43 and through resistor R45 or the transistor Q7, depending on whether the transistor Q7 is conducting as described below, to charge the capacitor C17. The return current path is through the circuit ground of capacitor C17 to pin 5, indicated at 23 and then to diode D3 and resistors R78 and R79. The inverted triangles (e.g. ground connection 21 in Fig. 1A) throughout the schematic circuit diagram shown Figs.
1A, 1B and 1C represent circuit ground connections. During this time, diodes D2 and D4 are reversed biased and are therefore turned off. The resistors R78 and R79 can be selected to limit current to 5.0 mA when the relay Ki is OFF to allow actuation of LEDs 24 and charging of the capacitor C17, among other power supply functions. When the relay Ki is closed, the current flows through the relay to the load 12.
When the polarity of the input power changes so that the line signal is at a negative potential with respect to ground, diodes D2 and D4 conduct and diodes Dl and D3 are reversed biased or turned of f. Current f lows f rom ground to inductor Ll, resistors R64, R68, R69, diode D4, resistor R52 and the emitter-base junction of transistor Q9 to charge the capacitor C17 and bias the diode D9. The return path is through the ground connection of the capacitor to diode D2 via the pin 5. This current path occurs on the negative half-cycle regardless of whether the relay K1 is ON or OFF.
The combination of using two separate half-wave rectifier circuits, which operate at opposite polarities and on alternate half-cycles of the input power, provides full-wave rectified power to the circuit 10, yet eliminates interaction between the line-to-load and line-to-ground current paths since the half-wave rectifier circuits are not conducting simultaneously. The isolation of the line-to-load and line-to-ground current paths is advantageous when, for example, safety requirements restrict leakage currents in one or both paths. The use of two half -wave rectifier circuits reduces the likelihood that current leakage in one of the paths contributes to undesired current flow in the other path.
With reference to Fig. 1A, an inductor L1 is placed in the line-to-ground current path to reduce the effect on the circuit of any noise present on the AC line. The resistors R64, R68 and R69 limit line-to-ground current to a selected, predetermined level, e.g. 0.5 milliamperes (mA). The transistor Q7 normally operates in a conductive state. The microcontroller 22 is programmed to turn on the transistor Q8 when the switch SW1 is switched to the OFF position. When transistor QB is in a conducting state, the transistor Q7 is turned off. The line-to-load current is then limited by resistors R45, R78 and R79 to a preset limit (e.g. 0.5 mA). Thus, when a person servicing the lighting fixture, for example, switches the circuit 10 to OFF via switch SW1 (Fig. 1B), the line-to-load current is limited for safety reasons by opening relay Kl. This is necessary in the event that the relay K1 was not switched to an OFF position by the microcontroller or was operating in a daylight override mode where current is not limited.
In accordance with an embodiment of the present invention, a transistor Q9 (Fig. lA) is provided having a base connected to the resister R45. Any time current flows through the line-to-ground current path, the transistor Q9 is placed in a conducting state. Accordingly, a transistor Q10 is placed in a conducting state. The transistor Q10 generates output pulses as long as the ground conductor is connected and line-to-ground current flows. The microcontroller 22 is programmed to monitor the output of the transistor Q10 for the presence of pulses.
When the ground is not present, no pulsea are detected by the microcontroller and the microcontroller 22 is programmed to generate an audible tone via the buzzer BZ1 and/or give a visual indication that a problem exists with the ground connection (e.g. illuminate a light emitting diode (LED) 24 in an LED
driver circuit 26). Thus, service personnel receive an indication that the circuit 10 is missing or has an improper ground connection. Alternatively, the microcontroller 22 can be programmed to annunciate when a ground connection is present.
The ground connection monitoring process of the microcontroller 22 is preferably performed during a start up routine for the system 8, but can occur when the system 8 is running and the ground connection becomes open. If the lighting fixture is turned on, the microcontroller 22 can sense an open ground connection, turn off the load and annunciate in a manner similar to the start-up routine. The program code for controlling the microcontroller in accordance with the start up routine is provided in Appendix A and will be described in connection with the flow chart in Fig. 2.
With continued reference to Figs. 1A, 1B and 1C, the power supply circuit 10 comprises a voltage regulator VR1, the input of which is monitored by the microcontroller 22 in accordance with another aspect of the invention. The circuit 10 comprises a large capacitor C32 (as shown in box lOB in Fig. 1C). A
transistor Q11 is activated via the microcontroller 22 to apply a pulse current to the capacitor C32 to charge the capacitor.
While the capacitor C32 is charging and Q11 is ON, the amount of available supply voltage decreases. The microcontroller is programmed to actuate transistor Q11 when sufficient supply current is available. The sufficiency of the supply current is determined via a feedback loop wherein the microcontroller monitors both the voltage regulator VR1 input and the capacitor C32. The microcontroller compares the voltage regulator input with threshold values which are variable within the program code. If the voltage regulator input is above a threshold value (e.g. 8 volts), the transistor Qil is actuated via a microcontroller output signal. If the voltage regulator input is below the threshold value, no modulated pulses are generated by the microcontroller to replenish the charge on the capacitor C32. The voltage on the capacitor C32 is then determined by the microcontroller, which activates transistor Q11 again if more capacitor voltage is desired and sufficient supply current is available.
By monitoring the voltage regulator input, the microcontroller self-regulates the voltage available from the capacitor C32, that is, the ability to charge the capacitor depends on the energy in the circuit, as detected at the voltage regulator input. This eliminates the possibility of charging the capacitor C32 using pulses with widths that are too large.
The microcontroller is also programmed to selectively turn off certain high current loads such as the LEDs 24 to facilitate rapid charging the capacitor C32. For example, selected components can be turned off if the capacitor C32 threshold, which is variable in the microcontroller program code, falls below a predetermined threshold (e.g. 3.7 volts). Further, if the voltage at the voltage regulator input is below a minimum threshold (e.g. 6 volts), the microcontroller is programmed to open the relay Ki to power down the load. This ensures that when power is restored, the capacitor C32 can be charged rapidly to begin operation of the system 8.
The microcontroller 12 has an output RCO for providing switched power to devices having higher current requirements relative to most of the components in the circuit 10. For example, a photocell 34, a resistor R54 for time out adjustments and an EEPROM 38 are provided with switched power from the microcontroller 22, that is, pulse power verses steady state power in accordance with a signal generated by the microcontroller and in conjunction with the microcontroller 22 requiring use of these components.

.

The microcontroller 22 is connected to a motion sensing circuit 30, a relay driver circuit 32, a photocell 34, the LED
driver circuit 26, the EEPROM 38, an external input device interface 40 and a number of push button switches, such as a LIGHTS switch SW3, indicated at 42 and a DAYLIGHT SET switch SWS indicated at 44.
With reference to the flow chart in Fig. 2, the microcontroller 22 is preferably programmed in accordance with a start-up routine to charge the capacitor C32 (block 50).
The microprocessor then opens the relay K1 to ensure that the load (e.g. the lighting fixture) is off (block 52). This is advantageous because the relay K1 may have been left in the ON
position when the power supply circuit was shipped from the manufacturer, or because power to the power supply circuit 10 was interrupted when the power supply circuit was operating with the relay K1 in the closed or ON position.
The microcontroller determines the voltage on the capacitor C32 via one of the analog-to-digital (A/D) converting inputs (e.g. RA3) on the microcontroller (block 54). If the capacitor voltage is less than a predetermined threshold (e.g. 8 volts), the microcontroller determines the voltage at the input of the voltage regulator VR1 via another A/D converting input (e.g. RA5), as shown in block 56. If the input voltage in below a predetermined threshold (e.g. 8 volts) , then the supply current is not deemed to be sufficient to charge the capacitor. The microcontroller therefore does ....

not activate the transistor Q11 to charge the capacitor, as indicated by the affirmative branch of the decision block 58.
If sufficient supply current is available (e.g., 8 volts), the microcontroller supplies a pulse current to the capacitor C32 via the transistor Qll to replenish the charge on the capacitor C32, as shown in block 60. As indicated by the negative branch of decision block 62, the microcontroller monitors the capacitor C32 voltage and the supply voltage and continues to charge the capacitor, provided sufficient supply current is available, until a desired capacitor voltage is .obtained (e.g.; capacitor C32 voltage is substantially equivalent to the V1 voltage in the system 8).
With continued reference to Fig. 2, the microcontroller is then programmed to determine if output pulses are being generated by the transistor Q10 (block 64). If they are not being generated, then the microcontroller generates a tone via the buzzer BZ1 andlor drives an LED 24 (block 66). Thus, an operator (e.g., a repairman) is notified that the ground connection is missing and that precautions need to be taken when handling the power supply circuit (e.g., wall switch unit for controlling a lighting fixture).
While certain advantageous embodiments have been chosen to illustrate the invention, it will be understood by those skilled in the a,rt that various changes and modifications can be made herein without departing from the scope of the invention as defined in the appended claims.

AMancGx A
S'TARTpP 1F.STS
LOC OB7F.CT CODE
VALUE LM SCIURCE TEXT

0560: Oi- startup, cbarge BiSCap.

0564: Bi4BCap START[JP::

0043 16A2 0571 skupO bcf SYSTAT, B-J.K'HT ENSURE that lights go off 0572 sNupl DI Inte=Upts off 0044 1368 M bcf INirCON, t3IE IRPS OFF

0045 1368 M bcf INT'CON, CiIE IRPS OFF
0046 0064 0574 clrnrdt 0p47 0576 STARIFS'I
0577 AtoO CBIGCAP
OU47 141F M bcf ACCONO, ADON :llirn!-/D power on 0048 3009 M mc>ulw CBIGCAP
0CA9 009F M mavwf ADCONO Sdcct cbarmei c04A 0000 M tyop : Let CBIGCAP sctik 0048 0000 M mp 04dC 151F M bcf ADCONO, GO-DONE Start coaversion 004D 0000 M nop 004B 0000 M mp 00v 091E M ux-vfw ADRES Get A/D result 0050 IOIF M bcf ADCONO, ADON 'Ilnm A/D power off c051 3CF4 0578 sublw BIGCAP THRPSH-JHI : Wait till we're WAY up thao 0052 1C03 2867 0579 BNC powrrou Up to spatd Atpee-da A
STAIt[UP'ITsSTS
LOC OBJECT CODE
VALUE T.INE SOURCE'1'E7CT
~.~.
0580 AtcO CPSUNREG Check iacoming powa 0054 141F M bcf ADCONO, ADON Turn A/D power on 0055 30B1 M movlw CPSUNREG
0056 009F M inovwf ADCONO Scloct channol 0057 0000 M ncp : Let CPSUNRF.G sottla 0058 0000 M ncp 0059 1S1F M bcf ADCONO, GO_J)ONB Start oanvusion 005A 0000 M ncp 005B 0000 M ncp 005C 08 IE M movfw ADRES Get AID resutt 005D IOIF M bcf ADCONO, ADON 1mn AID power off 005E 3C33 0581 sublw PStJNRBC3 TARESIi_I.o THEtESH - AtcO Go tM we gct law:

00SF 1C03 0583 Sm 0060 1097 0584 bcf PSPOICT, B CAPf3ll'TB Above tbtcalt, CLacge BigCap=

0062 1487 0586 bcf PSPORT, B._CAI'(3A'lE Default o8:

0064 1507 0589 bcf BEEPOItT, B-BFEPWM Activity stoisc whan cbargmg 0065 1107 0590 bcf BEff?PO1tT', B-MmpSVM
0066 2847 0591 goto STAICIBST

0067 0593 porreran.
0067 1EA2 0594 btfoc SYSTAT, B_IJGI[1' Tbm Bgltts off dt t~e 0068 286C 0595 goto poal Alreadp doae, Cbatiooe 0596 . El Typc on for RYOFF
0069 1786 M bcf IN'TCON, GIE II{PS ON
006A 2341 0597 caII RELAYOFF Lights off 1st time 006B 2844 0598 goto slnpl If.ick C one mora time.

006C 0600 ponl:

0602 : Check for gnxiod, Interrupts sbould still bo off bm.

006C 3020 0604 maviw 32 20ox = 8 * 160, @ 5 ryclcsJspin 006D OOiO 0605 movlf ACO
006E 0064 0606 ckwdt 006F 1E87 0608 pon2 btfss PSPOR'>; BL-GNDCK
0070 207C 0609 goto pon3 Got x low. Wc'rc CSI
0071 08FU 0610 dxfa ACO
0072 286F 0611 goto pon2 0613 : Ttuwd out wl no low on RCS, NO aROUPI Baep & restart.

0073 3002 0615 nuorlw 2 0616 sctbaok 1 M if 1 ** 0 ld bcf S1'AIUS. 5 Ckar bank bit @~amk 0) M endif M if 1**l.
0074 1683 M bcf STATUS, S
M cnd'if 0075-. 0092 0617 movwf FR.28t37F : Cvoack for bad gzotwd. (.5 aoranl5eq) 0618 sctbauk 0 M if 0=*0 076 1283 M bcf STAIIIS, 5 Clear bank bit. (Bank 0) M endif M it 0'*I
M bcf STATUS, S
M c fif 0619 F,I
077 1788 M bcf INICON, GIS : IRPS ON
078 2300 0620 caU BEEPI
0621 T'I
079 1388 M bcf IMON, GIE IRPS OFF

O1A 1388 M bcf WDCON, GIB IRPS OFF
07B 2878 0623 pon2a goto pon2a

Claims (18)

1. An apparatus for monitoring a ground connection in a three wire power supply circuit which selectively connects and disconnects a load from an alternating current power source having line, neutral and ground conductors, said ground connection monitoring apparatus comprising:
generating means for generating an output signal;
control means connected between said line conductor and said ground conductor and connected to said generating means, said control means being configured to operate said generating means to generate said output signal when current flows between said line conductor and said ground conductor and to interrupt operation of said generating means when current does not flow between said line conductor and said ground conductor; and indicating means connected to at least one of said control means and said generating means and operable in response to said output signal to indicate at least one of the presence and the absence of current flow in said ground conductor.

2. The ground connection monitoring apparatus as claimed in claim 1, wherein said control means comprises a processor connected to said generating means for monitoring the output of said generating means for the occurrence of said output signal, and for controlling operation of said indicating means when said output pulse is not detected.

3. The ground connection monitoring apparatus as claimed in claim 2, wherein said processor is programmable to commence monitoring the output of said generating means when said power supply circuit is powered on using said power source during at least one of starting up said power supply circuit, and restarting said power supply circuit after an interruption in power from said power source.

4. The ground connection monitoring apparatus as claimed in claim 1, wherein said control means comprises a processor connected to said generating means for monitoring the output of said generating means for the occurrence of said output signal, and for controlling operation of said indicating means when said output pulse is detected.

5. The ground connection monitoring apparatus as claimed in claim 4, wherein said processor is programmable to commence monitoring the output of said generating means when said power supply circuit is powered on using said power source during at least one of starting up said power supply circuit, and restarting said. power supply circuit after an interruption in power from said power source.

6. The ground connection monitoring apparatus as claimed in claim 1, wherein said generating means comprises a first transistor, and said control means comprises a second transistor having first and second terminals thereof connected to said line conductor and to said ground conductor, respectively, and a third terminal thereof connected to said first transistor, said second transistor being biased into a conducting state when current flows in said ground conductor, said second transistor being operable to bias said first transistor into a conducting state.

7. The ground connection monitoring apparatus as claimed in claim 1, wherein. said indicating means is selected from the group consisting of an audible indicator and a visible indicator.

8. An apparatus for monitoring a ground connection in a three wire power supply circuit which selectively connects and disconnects a load from an alternating current power source having line, neutral and ground conductors, said ground connection monitoring apparatus comprising:

a generating circuit coupled to said line conductor and said ground conductor for generating an output signal when current flows between said line conductor and said ground conductor;
a processor connected to said generating circuit and programmable to monitor the output of said generating circuit for the occurrence of said output signal; and an indicator connected to said processor, said processor being programmable to control said indicator to indicate a condition selected from the group consisting of the absence and presence of current flow in said ground conductor.

9. The ground connection monitoring apparatus as claimed in claim 8, wherein said processor is programmable to commence monitoring the output of said generating circuit when said power supply circuit is powered on using said power source during at least one of starting up said power supply circuit, and restarting said power supply circuit after an interruption in power from said power source.

10. The ground connection monitoring apparatus as claimed in claim 8, wherein said generating circuit comprises a first transistor, and further comprising a second transistor having first and second terminals thereof connected to said line conductor and to said ground conductor, respectively, and a third terminal thereof connected to said first transistor, said second transistor being biased into a conducting state when current flows between said line conductor and said ground conductor, said second transistor being operable to bias said first transistor into a conducting state.

11. The ground connection monitoring apparatus as claimed in claim 8, wherein said indicator is selected from the group consisting of an audible indicator and a visible indicator.

12. A method of monitoring a ground connection in three wire power supply circuit which selectively connects and disconnects a load from an alternating current power source having line, neutral and ground conductors, said method comprising the steps of:
switching a circuit element in a current path comprising said line conductor and said ground conductor into a conducting state to generate an output signal when current flows through said current path, said circuit element operating in a nonconducting state and generation of said output signal being interrupted when current is not flowing through said current path;
monitoring said circuit element for said output signal to determine when said circuit element is in a conducting state; and operating an indicator when said circuit element is in one of a conducting state and a nonconducting state.

13. The method of monitoring a ground connection as claimed in claim 12, wherein said operating step comprises generating an audible tone.

14. The method of monitoring a ground connection as claimed in claim 12, wherein said operating step comprises operating a visible indicator.

15. The method of monitoring a ground connection as claimed in claim 12, wherein said monitoring step and said operating step occur when said power supply circuit is powered on using said power source during at least one of starting up said power supply circuit, and restarting said power supply circuit after an interruption in power from said power source.

16. The method of monitoring a ground connection as claimed in claim 12, wherein said switching step comprises the step of biasing a first transistor connected to the line conductor and the ground conductor into a conducting state.

17. The method of monitoring a ground connection as claimed in claim 16, wherein said monitoring step comprises the step of monitoring the output of a second transistor connected to said first transistor and operable to conduct when said first transistor is conducting and to generate output signals, and said operating step comprises the step of operating said indicator when said output signals are detected.

18. The method of monitoring a ground connection as claimed in claim 16, wherein said monitoring step comprises the step of monitoring the output of a second transistor connected to said first transistor and operable to conduct when said first transistor is conducting and to generate output signals, and said operating step comprises the step of operating said indicator when said output: signals are not detected.