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problem circuit

The above circuit has two sections.

Section A is a simple transistor as a switch circuit using BC547 NPN transistor which triggers a LED1 as well as section B.

Section B is a NOT gate circuit using BC557 PNP transistor which triggers another LED2.

Target: when push button is on LED1 should glow while if push button is off LED2 should glow.

Issue: when push button is off LED2 is glowing as expected but LED1 is also glowing with lesser brightness.

I understand the reason that the current is flowing through the pnp transistor's B-E and completing the circuit. But this is not expected in the circuit.

Can anyone suggest what is the solution to this?

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    \$\begingroup\$ Welcome! Please show where the current is flowing in reverse. \$\endgroup\$
    – winny
    Commented Jun 12, 2022 at 14:20

2 Answers 2

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I don't see any problems. You likely chose the resistor values incorrectly. Always show all details of your work. We can't tell you what you did wrong if you don't tell us what you did!

schematic

simulate this circuit – Schematic created using CircuitLab

To see this in action, run the simulation, highlight SW1 by clicking on it, then press Space to toggle it. Observe the current meter readings. The LEDs will not be visibly glowing much when the current is <20uA.

The only disadvantage of this circuit is that it wastes energy by shunting the LED, i.e. when D1 is on, it uses more current than when D2 is on.


I understand the reason that the current is flowing through the pnp transistor's B-E

That's not the reason. This current can not light up D1: it flows the wrong way. It also can't turn on Q1.

The only way what you claim would be true is if you had a circuit like below:

schematic

simulate this circuit

Here, you can clearly see that D1 gets driven by the B-E diode of Q2. But this doesn't happen in the circuit you've shown!


The circuit below uses a long-tailed pair and works quite well. The main disadvantage is that the input is not differential, and thus there's residual imbalance. About 0.5mA less flows through D2 than D1. There is a tradeoff between dissipation in R1,R4 and the balance: the lower the values of R1,R4, the better the balance of the LED currents. As it stands, the circuit would be acceptable for many uses as an indicator.

schematic

simulate this circuit

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  • \$\begingroup\$ Hi, the 3rd circuit is working fine for me. Thank you for your help and detailed schematics. \$\endgroup\$
    – p2013
    Commented Jun 17, 2022 at 17:39
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A NOT gate is not a gate. Instead it is a logic inverter. Your PNP transistor was not a logic inverter, instead it was an emitter follower.

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    \$\begingroup\$ I would say that “an inverter is not a gate” is not a helpful statement in any way. It explains nothing and I’m not even sure it being true would be of any help. “Here’s all the various logic gates. Oh, and the inverter, which does a combinatorial logic function but is not a gate.” Like, whaaaaat? \$\endgroup\$ Commented Jun 12, 2022 at 18:33

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