Monostable 555 Timer Switch Question

Thread Starter

jmacusaf

Joined Jun 11, 2014
5
Hello Everyone,

I am very new at electronics and was hoping someone can give me a quick solution or a place to look since my research has come up short.

Is there a way to trigger a one-shot infrared LED in a 555 Timer circuit with a momentary switch continuously depressed?

I have read that the amount of time a switch is depressed has to be shorter than the pulse. In my project, I would like a magnetic connection to press and hold a momentary switch to trigger a one shot infrared LED flash. However I do not want the battery to drain if the pulse is just going to keep happening while the switch is depressed. Not really sure where to look. Every schematic and tutorial does not mention a continuous momentary switch press.

Thank you for your help
 

MikeML

Joined Oct 2, 2009
5,444
If I understand your requirement, you want a short pulse every time a switch is operated, the duration of the pulse being a short pulse, even if the switch stays depressed for a long time?

What are the current requirements for your LED?

How long a pulse do you need?

What is the supply voltage?
 

Thread Starter

jmacusaf

Joined Jun 11, 2014
5
Thats correct. A 2 second pulse operated by a 3-5v battery that only pulses 1 time even while the switch remains depressed. Its mainly to reserve battery life.

I did not buy the infrared LED emitter yet but it will be a typical one from radio shack - Forward voltage (V): 1.28V, rated at 100mA
 

crutschow

Joined Mar 14, 2008
34,201
......................
BTW, that circuit starts when S1 is released.
No, a 555 starts when the input trigger goes low.
But the circuit shown in post #4 will retrigger and the output will stay high as long as the switch is depressed.
You need to capacitively couple the input trigger to pin 2.

Another problem is that a standard 555 will not operate at 3V.
You could use a CMOS LMC555 which can operate down to 1.5V. But since the output current capability of that is limited to a few mA, depending upon the supply voltage, you would also need to add a transistor buffer to drive the LED.
 

MikeML

Joined Oct 2, 2009
5,444
Here is my idea. Works on 3V. Drives a grounded-cathode IR LED. Uses parts available from RatShack.

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D1-D2 represent the IR LED. I didn't happen to have a model that had the 1.45V Vf, so two Si diodes in-series comes close. R4 sets the LED current, but with only a 3V battery, there is not much headroom for good current regulation.

Time delay is primary determined by the time-constant (R2//R3)*C1. Leave R2 and R3 alone, and change C1 to make it different.

D3 and R1 discharge the timing capacitor while S1 is open, so that the one-shot is ready for a new cycle.

There is zero standby current drain with S1 open.
 
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