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schematic

Circuit simulation is here

There are two parts to this circuit. The first part is a switch that is either floating or grounded, controlling a large number of relays via pnp transistors. (Many relays are omitted but imagine they are there, in parallel) This part of the circuit is working perfectly. The second part is LED indication. One LED should be on if the switch is thrown, the other should be on if the switch is not thrown.

This part of the circuit almost works properly, but the problem is that LED "B" glows faintly when the switch is thrown.

How do I modify this circuit so that LED "B" turns off completely?

(The switch is fixed, replacing it is not an option.)

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    \$\begingroup\$ You really should have a pull-up resistor on your switch. Leaving the transistor base floating could allow the transistors to turn on unintentionally. \$\endgroup\$
    – HandyHowie
    Commented Aug 6, 2021 at 8:51
  • \$\begingroup\$ Thank you, I will add that. \$\endgroup\$
    – cat pants
    Commented Aug 7, 2021 at 6:42

1 Answer 1

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A simple fix is adding a pull-down 1k to base of B.

The A emitter voltage was almost high enough to enable B.

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  • \$\begingroup\$ Thank you. I tried this, but unfortunately it causes LED "B" to dim quite a bit when it's on. Is there any way to fix this circuit? I don't mind having to change the transistor topology, resistor values, etc, I just can't change the switch or the relay part of the circuit. \$\endgroup\$
    – cat pants
    Commented Aug 7, 2021 at 6:43
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    \$\begingroup\$ Try nearest values to these if using Red. tinyurl.com/ygq7ekc3 \$\endgroup\$ Commented Aug 7, 2021 at 13:31
  • \$\begingroup\$ Thank you so much!! \$\endgroup\$
    – cat pants
    Commented Aug 7, 2021 at 22:28

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