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555 Timer Circuit using a Toggle Switch


New Member
Hi there. Hoping I can get some help as after 2 days of trying and giving myself a headache I need some help.

To start, I'm just a hobbyist with far too little time on my hands but with a basic level knowledge of electronics but willing to put in the time to improve my learning.

To give background, to make a perimeter alarm a Personal attack alarm can be used with a string tied to a piece of card holding apart two sides of a cloths peg (Which had draw pins and wire to act as an electrical contact switch) which breaks the connection between the battery and the battery contacts of the alarm. Once the plastic card is pulled out the alarm triggers.

However, this will make the alarm ring indefinitely until the batteries die or the pin is replaced. Because of this for the games it will be used in I would like to create a circuit which will allow a trip wire to activate a 555 timer which will allow the alarm to ring for 15 seconds before turning off. This I can do with a tactile switch, but as the trip wire switch (cloths peg with contact surface held apart by plastic card) is a SPST normally open switch I cannot for the life of me get it to work; as replacing a tactile switch with this toggle switch produces the same result as not having a 555 at all.

Once I get the above problem solved the next challenge I want to tackle is making the circuit that once it has been triggered it will not trigger again until reset by powering off the unit or reset button. (initially thinking of a second 555 timer which can set the time period of the lock out of the 1st 555 to 24hrs or longer).

I Hope I've explained this clearly enough but to summarise the desired function of the circuit:

- Normally open toggle switch once triggered will activate 555 timer
- 555 timer will allow alarm to ring for set time (ie 15 seconds) and stop even with toggle switch remaining closed.

-after 15 seconds alarm will stop and require a turning off and on again before being able to reuse.

Any and all help would be greatly appreciated.

Many thanks,



Well-Known Member
Most Helpful Member
To your advantage, a 555 triggers on power-up unless extra effort is taken to avoid it. So, simply setting a mono-shot layout (triggered once) and using your card/clothespin switch at powerup (power to pins 4 and 8) will work.


Well-Known Member
Most Helpful Member
It is problematic to tie the TRIG input low on a 555, since the inputs are level triggered and it will likely not time out (output stays high).

Below is the LTspice simulation of a 555 circuit which avoids that problem by connecting the TRIG input to the charging capacitor.
The capacitor voltage is initially zero, so when circuit power is first applied (yellow trace), the 555 TRIG input is triggered, the OUTput goes high (blue trace), and the capacitor starts charging (red trace).
This is similar to an astable configuration, but with no DIS connection, so when the capacitor charges to the THRS voltage, the 555 internal latch is reset to the low OUTput state (at about the 16.7s point on the simulation), but the capacitor isn't discharged.
The THRS and TRIG inputs thus stay high, which maintains the 555 internal latch in the low state, so it doesn't continue pulsing.

The result is a single 15 second output pulse (as determined by R2-C1) upon the application of power, and then the output stays low until the power is reset.

D1 discharges C1 when the power is removed.

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