I have two seperate sub-circuits that each have an indicator LED and only one circuit will have power at a time. So far so good, however I would much prefer to have only a single indicator light.

My Problem is that one LED comes out of a 555 IC to VCC and the other comes out of the emitter pin of a 2N2222A transitor and runs to GND.

If they both went to VCC or both went to GND it would be easy.

I tried adding a 2N2222A transistor to the 555 IC (Base to pin), collector to 9VCC and running the LED out of the emitter to GND. I also tried a few resistor values between the collector and the 9VCC.

The subcircuit still worked, but the indicator LED would not turn on.

Is there any simple way this can be done?

  • \$\begingroup\$ Is it part count or pcb space that is the issue? \$\endgroup\$ – Passerby Feb 24 '15 at 2:58
  • \$\begingroup\$ May we have a schematic or two of what you have tried, please? \$\endgroup\$ – Jerry Penner Feb 24 '15 at 4:44
  • \$\begingroup\$ Where does it come from the 555 timer ? What pin ? \$\endgroup\$ – efox29 Feb 24 '15 at 6:13

If you do not mind two transistors then the general principle implements
A OR NOT(B) at the LED Cathode.

The circuit below should work with both inputs as logic level signals with not enough drive for an LED (or with "full power" signals such as you have".)

A low turns Q1 on via R2.
R1 holds Q2 off if A is high and B is low as the base of Q2 would otherwise be floating - which is not good design practice and can lead to partial turn on of transistor due to leakage currents. Similarly Q1 may benefit from a high value resistor (say 100K ) from base to V+ IF input A is high impedance when not low to enable LED.


simulate this circuit – Schematic created using CircuitLab

____________________________enter image description here

Either can be used to drive the LED directly (as now) with the other inverted to match.
eg here A could be connected to the collector of Q2 via a diode and R4 and Q1 omitted.

D2 may not be needed depending on impedance of B when not on.

A low and/or B high turn on LED.

  • \$\begingroup\$ You may want to enhance your answer to show how to provide bias for the output stage from which ever of the OPs two power rails which may be on. \$\endgroup\$ – Michael Karas Feb 24 '15 at 6:58

Thanks for the suggestions guys

I have just solved this myself.

The first beam breaker circuit used a 555 timer and the status of alignment and sensitivty was originally shown by an LED (and 470R) between its pin 7 to 9VCC.

However I wanted the LED to run to GND to match a second beam breaker circuit's (built with only transistors) indicator LED. Then they could be amalgamated.

After much faffing around with the second circuit and getting nowhere, I had another go with the first circuit that used the 555 timer.

The solution was to replace the 555 pin 7 to LED (and 470R) to 9VCC connection with a PNP transistor; I used a 2N2907, connected as follows: 555 timer Pin7 to Base, Collector to 9VCC and Emitter to LED (and 470R)and then to GND.

(An NPN is out of sync, off when it should be on, on when it should be off)

Elegant simplicty.

Thanks again.


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