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.
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.
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