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I have build a latching circuit using both a NPN & PNP BJT to turn on and off an LED, as shown below:

schematic

simulate this circuit – Schematic created using CircuitLab

When I built the circuit and tested it, the LED does not light.

When the ON BUTTON is pressed, the voltage across the collector-emitter of Q1 is 9V, meaning the BJT is not conducting to pull the base of Q2 to ground.

I have tried making the value of R3 much smaller (even down to a 2.2k resistor) to where the base current in Q1 was 170uA, but the BJT still won't conduct.

I have left the OFF BUTTON out of the circuit in case it was holding the base low, it is open circuited now.

I have tried a few of the same BJT's which made no difference and I am sure the circuit is wired correctly.

Has anybody got any suggestions ?

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    \$\begingroup\$ Would you prefer a circuit that only requires ONE push-button instead of two? Or is two what you want? \$\endgroup\$ Commented Aug 29, 2019 at 18:23
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    \$\begingroup\$ Have you measured the voltage at the base of Q1 with the button pressed and not pressed? You might want to put a current meter (I'd put it in series with a 100\$\Omega\$ resistor, just in case) from the collector of Q1 to ground (i.e., short Q1 and see how much current flows) -- it should be less than a mA. \$\endgroup\$ Commented Aug 29, 2019 at 18:27
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    \$\begingroup\$ @David777 I didn't look at the circuit when asking. I just wanted to be sure about your goals before wasting time looking it over. So first glance right now, the first thing that bothers me is your OFF button using a \$100\:\text{k}\Omega\$ to a collector node. But then looking more closely I now see that it's tied to a base, too. I'll look more, shortly. But it really needs to be re-drawn. \$\endgroup\$ Commented Aug 29, 2019 at 18:46
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    \$\begingroup\$ There's either a wiring problem or your switches aren't switching. Try either measuring the switch resistance with it closed, or try shorting the switch contacts, or both. If the switch has more than two leads, some of them may just be there for mounting, not for actual switching. \$\endgroup\$ Commented Aug 29, 2019 at 19:43
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    \$\begingroup\$ Guys, I hate to admit it, but it was indeed a wiring mistake. Thank you all so much for your help. \$\endgroup\$ Commented Aug 29, 2019 at 21:43

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