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Here's a circuit I'm trying to get working :

enter image description here

I have two digital signals (B and C in the diagram) coming from a CD40174 flipflop. I use these to light up two LED's.

I also use these as the input for a transistor based NOR gate the output of which I call A.

I also need to be able to drive an LED from A

The problem is that if I calculate R11 (the base resistor for the LED driving transistor) correctly (about 5K) the voltage at A is significantly less than 9V.

I am using A elsewhere and it needs to be as near 9V as possible. Also I'd like to draw as little current as possible from the flipflop outputs.

If I raise R11 to 100K I get a nicer, very nearly 9V at A, but the current through the LED (on pin 8) is significantly less than 20mA

Can someone tell me how to resolve this?

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  • \$\begingroup\$ Also, modern LEDs can be pretty bright with just 2 or 3 mA current. Are you for some reason stuck using these crummy old ones that need 20 mA? \$\endgroup\$ – The Photon May 27 at 2:00
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If I understand your requirements correctly, you can just connect the pin 6/7/8 transistor as an emitter follower instead of common-emitter.

schematic

simulate this circuit – Schematic created using CircuitLab

This will only put ~7-8 V on the anode of the LED, so if you need this LED to be equally bright as the others in your circuit, you may have to adjust the R1 and/or R4 values down a bit.

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  • \$\begingroup\$ Wow.. Thank you very much.. That looks great. I'll give it a shot. \$\endgroup\$ – Richard May 27 at 9:08
  • \$\begingroup\$ Yes.. That works.. I just need to tweek the LED resistor limit values a bit. Obvious when you see it.. But as you can probably tell, I was just taking the common emitter design from somewhere else without thinking. Thanks. \$\endgroup\$ – Richard May 27 at 9:41

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