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Switch and Transistor Control

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Full Member level 6
Aug 1, 2009
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Need some help on the following circuit

When switch pressed Q1 will turn ON as I will make MC high, Q2 will also be ON and load will have 3.3V

When I release the switch, I want Q2 to be off... while Q1 stays ON as I would be controlling it by MC

I am not sure how i can make Q2 go low when switch released and again high on next press

Q1 should stay ON (that controls other part of circutry) until i make MC low

Any suggestions??

I am ok to use DPDT switch but a smd type with momentory press but unfortunatly couldnt find such part

At the moment, closing the switch may damage Q2, you apply the 3.3V directly across the B-E junction with it in conducting direction. You need a resistor in the base connection to Q2 to limit the base current.

There are several solutions, the easiest is probably to add two resistors and a diode.
1. Wire the diode and one resistor in series and fit them between the collector of Q1 and the base of Q2, diode should have it's cathode (+) end toward Q1.
2. Add the other resistor in series with the switch and connect it between ground (like it is on your schematic) and the base of Q2.

Q2 will then be controlled (but safely) as it is at the moment by the switch and Q1 will work as before as well as controlling Q2. I think that's what you want.



I want Q2 to be ON when switch pressed and OFF when switch released even when Q1 is ON

I am not sure how having a diode will help. ie if Q1 is ON, Q2 will be ON irrespective of switch condition

Below is the sch for your reference


If you want Q2 to be OFF when the switch is released even when Q1 is ON then Q1 does nothing.
Maybe you want to latch Q2 to stay on after it has been turned on?

The requirement is as follows

Note: I need to have a single switch only

1. On first press, a low output is needed and it should remain low, even when released (which i will do by enabling Q1)

Q2 should be off... But it is fine if its on when switch is pressed (if it can stay off on first press... great) but because I turn on Q1, Q2 also stays ON which shouldnt happen.

2. On second press, Q2 should go ON

and I will make Q1 off

and the process repeats.

Basically on first press Q1 should be ON and stay ON even when switch released

On second press Q2 should go ON and stay ON as long as switch pressed...

I hope I havent confused things :)

It sounds as though you need a circuit with a memory. An R-S flip flop is the basic example of this.

To get the action you want, consider trying a toggle flip flop, or bistable multivibrator.

I tried simulating.

Why does the MOSFET drain have 2.36V under given scenario

BradtheRad... I at the at first press might have only 7.5V input and hence using flipflops that need 5V or 3.3V arent feasible.... Let me anyways check the possibility

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