Project: Alarm Timer

Thread Starter


Joined Aug 8, 2005
Attached is a schematic for an old design I did to create an alarm for my motorcycle. I mounted the board under the seat (which locks), and attached the output to the bike's horn (I also had a small "Sonalert" under the seat). Whenever I parked and wanted to arm the alarm, I would open the seat and close the power switch (SW1), and then adjust the mercury switch to account for the angle of the bike (which can vary with where one parks and how the kickstand is angled).

My criteria at the time for designing this was no power usage (no battery drain) when it was armed, and to be rather small and compact so it would fit into the small area underneath the locking seat.

Circuit theory:

Provided the mercury switch SW2 is open: setting SW1 to the "closed" position arms the device by supplying "potential" power to the coil of K2 and the SCR Q1. The 555 timer is not powered because K2 is de-energized, and neither is K1 energized because pin-7 of the unpowered 555 is an open-collector output.

If, or when, the mercury switch SW2 is jiggled enough to make contact, it fires Q1 which energizes K2 and sends 12V to the bike horn (or any other noise devices that may be connected). The now energized K2 will also supply 12V to the 555 timer, but pin-7 will remain open until approx 25 sec later (1.1RC) when C2 charges sufficiently to cause pin-7 to go low and energize K1. When K1 is energized, it interrupts the power to Q1 and to K2, this turns off the alarm (because Q1 inherently latches in the "on" state, it can only be turned off by interrupting the voltage to the anode). The time the alarm is on is determined by 1.1RC, which is 25 sec as shown.

Although power is removed from the 555 when K1 is energized and K2 becomes de-energized, the 555 will operate down to about 3V, which will keep K1 energized until C1 (Vcc) is nearly depleted, and this provides sufficient time for the system to stabilize back into the "armed" state.

Of course, if the mercury switch SW2 continues to make contact, then the entire cycle will repeat itself.

I used small ~2A signal relays, the pinout may vary with the type you use. Paralleling both contacts in the DPDT relay helps guarantee good contact, and can also help in current sharing. The snubbers across the contacts help reduce contact arcing, which could cause the contacts to stick.



Joined Jun 30, 2006
You can use a double pole single throw (DPST) normally open (NO) contact relay with a coil voltage suitable for your cycle. The circuit is controlled on/off by SW1. You use one set of relay contacts to control the horn. The other set of contacts short across the mercury switch.

When the mercury switch is energized the relay closes, one set of contacts applies power to the horn, the other set shorts across the mercury switch keeping the relay energized and the horn on. You can turn it off with SW1.

It takes three parts. ;)

It is possible to use two relays if you can't find a DPST. I think the relay in your diagram will work as it seems to have two NO contacts.

Thread Starter


Joined Aug 8, 2005
If I wanted the alarm to stay on, I would have simply run it onto a latch -- no big deal -- but this design resets itself.

Typically (90%) the bike gets bumped by someone messing with it or looking to close. The mercury switch acts like a momentary switch and the alarm will shut itself off after ~25 seconds, so it typically doesn't stay on and run the battery down (especially if I'm not close by).

The 20-25 seconds is sufficient to scare away most anyone due to the attention drawn by the noise, and notify me to take a look if I'm within earshot. But I know it should reset, so I don't have to run out to unlock the seat and play with switches.

If the bike were to be moved sufficiently upright, or knocked over, then the mercury switch may stay in the closed position and the alarm will repeat, which tells me someone was being a bit more, shall we say, aggressive.

Note that it's not that difficult to disconnect a bike horn, or mess with the wiring, which is why I also installed a "Sonalert" under the seat.
The Projects Collection is intended to submit and ask questions about completed projects. If you want help or advice on a suitable project, please post in the appropriate forum.


Joined Apr 27, 2007
The Projects Collection is intended to submit and ask questions about completed projects. If you want help or advice on a suitable project, please post in the appropriate forum.
The project seems complete to me, as stated in the description. All projects in this section are approved by a moderator, and not posted before aproval. Also, people are free to suggest improvementys on this project, as in any other section.

The rules don't state otherwise:


Joined Apr 15, 2009
Or you could wire it with transistors so that once the mercury switch has turned on the transistor it is shut off until the time delay has finished were the transistor will supply the 'voltage' back to the mercury switch(s), you coul also use a 555 timer using pin 2 where it becomes not in use after it has the rght voltage across it

Hope it helped?