Creating a timer

[Fire32NJetta]

Ok we may be getting a 3t22 for our firehouse to replace the dead modulator. Now i have been told that the controls for this 3t22 have been ghetto rigged to do hi-lo all the time. Last time i talked to my guys they want nothing to do with hi-lo. Is there a way to create a timer circuit say from like a 555 timer or something that will turn on and off for a minute or so to wail the siren.

[Justin]

Hell yes!

You could rig a timer up with a 555 chip, but because I'm more of a programmer rather than engineer, I'm more accustomed to PIC's because of the simplistic circuitry.

If I manage to figure out how to get the Harbinger controller working, Ill give you the schematic and code if you wish?

Then again, Siren Central have proper controllers and I've seen some pop up on eBay.

[Elliott]

555 timers are pretty easy to work with, they only use 2 or 3 resistors and one capacitor to form a delay period, and can be configured for one-shot, such as a 3-5 minute delay, or astable multivibrator mode such as driving a hi-lo alternating signal. If you need two or more timers/delays, a 556 is two 555's in one 14 pin IC, just common VCC and Gnd between the two.
Here is a tutorial on 555's.

http://www.uoguelph.ca/~antoon/gadgets/555/555.html

Here is a quick calculator for delay intervals.

http://ourworld.compuserve.com/homepage ... en/555.htm

The output is current limited (200 milliamps at 15VDC), so I would use a NPN transistor on the output with a 1K resistor to it's base and the emitter-collector as a switch to, say, a 6 to 12 volt relay. A 2N3904 or 2N2222A would work, and are extremely common. Then the small relay could in turn switch a heavier relay/contactor for the siren that has a 120 volt coil.

NOTE: When driving a DC Relay coil with a transistor, a diode should be wired in parallel in REVERSE polarity to the coil. When the relay is energized, the diode will be "invisible" to the current, but when the coil is de-energized, the back-EMF spike caused by the resultant magnetic field collapse will be shorted/clamped by the diode, and protecting the transistor from getting zapped. The coil is an inductor, and can spit out a higher voltage than it is supplied sometimes!

Anyway, there is a ton of free info out there. Drop me a reply or PM if you need any help.

[Robert Gift]

Thanks so much Elliot.
Great advice about reverse EMF from inductors.