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How would I wire a light-emitting device (not necessarily an LED) in the following schematic to indicate the input (High to run) is high? Would it be in parallel with R4? Before or after R3?

enter image description here

Thanks, Russell McMahon, for making the schematic for me.

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up vote 5 down vote accepted

There are various places.

  • if the source can supply the required voltage and current, you could place it between 'high to run' and ground.

The next step is 'after' Q2, but that has a switch in parellel, so your light will light up if that is pressed too. You could extend the circuit with a resistor or a few diodes to prevent this.

  • Q2 switches on when 'high to run' is high, so you could place the light between Q2's collector and V_battery.

Next is between the collector of Q1 and ground. Like the collector of Q1, it is also activated when the switch is pressed (and prevent this parallel activation is a bit more involved), so maybe that is not what you want.

One additional note: if you use a LED on any of those places, don't forget the series resistor.

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Could I put a incandescent light bulb in between R3 and the circuit input without to much waste heat? (max 40mA draw for input) – user46377 Jun 30 '14 at 15:18
@user46377 Yes you could and there would be essentially no waste heat (since the lamp would not light up). – Spehro Pefhany Jun 30 '14 at 16:07
@user46377 (can't you really come up with a better name?) You mean in series?? That is not one of the options I proposed. You can of course do so, but as Spehro says.... – Wouter van Ooijen Jun 30 '14 at 17:09
If you're planing on using an Arduino Uno (assumed from your previous question), you should never source or sink more than 20mA per IO pin or 200mA total (all pins combined). The 40mA figure you may have seen is an Absolute Maximum for the ATmega328, which means you should design to be well below that. – Ricardo Jun 30 '14 at 18:47
@Ricardo, this configuration only allows for 430uA to be drawn into the base of Q2 (actually only 423uA into the base), so that should not be a problem. This is valid assuming the output is 5V as it would be from the ATmega328. – sherrellbc Jun 30 '14 at 18:59

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