Momentary to latching

Grossel said:

The circuit should be able to work with a lot of other possible resistor values also.

If you take your time drawing a multiline voltage/time diagram, you'll prove for yourself that the circuit will work. Thats how I do it when I cannot se how a circuit is working. You don't need the voltages to be perfect, just enough to assume that would make any connected transistor to turn on.

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Sceadwian said:

You can learn a lot from simulating simple circuits like this in something like LTSpice where you can view the voltages at any point in the circuit.

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colin55 said:

A fault in the Latching Push Button circuit has been corrected in this circuit:
**broken link removed**


A short description of the reasoning behind the values of each component:
R1 and R5 are large values so that the 1u takes a while to charge.
R5 is large so that the electro takes a while to discharge.
R4 can be a large value as it is only required to provide a small current to turn on Q1.
R2 can be anything from 100k to 1M as it only required to turn off Q1 when no current is flowing and allow a voltage of about 0.6v to appear between the base and emitter when the transistor is required to be turned on.
R6 is a current limiting resistor to limit the current onto the base of Q3 when the switch is pressed and C1 is charged.
R3 and R4 are identical in operation to R4 and R2.
R8 limits the current into the base of Q2 when the circuit is turned on. This resistor was omitted from the original design and is MOST IMPORTANT.
When Q1 and Q2 are turned on, the current through these transistors (without the 10k) will be considerable, especially with a 12v supply and the two transistors will be destroyed.
Obviously the circuit has never been tested by the designer and that's why you must never put up a design without firstly testing it.

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