Converting latching switch to momentary output

Question

I'm trying to get a latch switch to send a pulse for around 3 seconds or so. The pulse doesn't need to be pretty or precise. The circuit will be used to sound a buzzer when the switch is flipped on. It shouldn't sound when the switch is released, as supposedly no current will be flowing from a 9V battery I'm strapping to it.

I know you can kind of accomplish something like this with a Schmitt trigger gate, maybe. It's been over 5 years since my electrical engineering course and I'm a bit stumped. I'd use a 555 IC if that'd work with a latch switch, but I'm not sure how to get that to work as I think it requires a momentary pulse from a button or something.

Here is a criminally bad diagram shows what I'm trying to do.

Any help is greatly appreciated.

Answer 1

If you can substitute for the SPST switch you show in your drawing, a SPDT switch, with one contact connected to positive, and the other to negative, it will almost certainly make your solution simpler.

Here is a possible solution for you. The timing of the circuit is not very accurate. If you need accuracy, let me know, and I will give you a more complex circuit. Instead, this circuit uses a variable resistor to adjust the duration of the alarm.

There is a resistor symbol in the schematic labeled ActiveBuzzer. As you may have guessed, it represents the active buzzer. An active buzzer will make a sound simply by applying a voltage. Some buzzers are passive, and require an external oscillator in order to produce a sound. You can ignore the resistance value. I simply used an arbitrary value in order to simulate the circuit.

The timing of the circuit, as I mentioned, is not very accurate. It depends upon both the state of battery charge and the threshold voltage of the mosfet. Threshold voltages of mosfets vary widely. You can, and perhaps in some cases need to, change the values of R2 and C1. The larger either is, the longer duration the alarm.

The circuit draws almost no current except when the buzzer is active.

If you cannot substitute a SPDT switch for the SPST switch, the circuit will be necessarily more complex.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 2

You are probably looking for a monostable circuit, which can either be accomplished with a 555 ic or with with a few transistors. Here is a small simulation using some bipolar devices.

The trigger input was replaced with an inverter in order to latch on the positive edge (switch on).