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BACK to the home page. - Jump to 4-stage Timer.

Update! 1/12/03 - I forgot to show a diode across the relay coil. See new schematic above ^

I tossed this one together due to many requests for a minimal part count timer - I can't really recommend it for trail-cams.

This is a deceptively simple looking timer at first glance. The theory is that the Capacitor "Cx" is rapidly charged from the output of pin #2 of the "big IC" of the RS 49-425 when motion is detected. The first 1N4001 diode prevents the IC's output from draining the cap. After charging, the cap slowly discharges through the resistor "Rx". While the cap is discharging, the gate of the 2N7002 (or similar) MOSFET is held "HIGH" and the relay or other device is turned on.

The use of a MOSFET with its extremely high gate resistance allows for low power consumption and LONG time delays. If you eliminate "Rx", the time delay will be determined by the internal leakage of the cap and the MOSFET gate leakage. This could be several hours if a very large cap is used. MOSFETs are static sensitive devices. Touch a grounded object (plumbing, electrical outlet, etc) occasionally while working on the circuit - or work at a "static-safe" work area.

Note the second 1N4001 diode added across the relay coil. This helps shunt voltage spikes when the relay is turned off. With out this diode the MOSFET could become damaged immediately or over time.

The rough (perfect world) math for this timer is something like this:
Time 'ON' (in seconds) = Cx (in micro Farads) x Rx (in Megohms) x 0.75     Example: 100uF x 1M x 0.75 = 75 seconds

But......In the real world we need to account for other items such as internal leakage through the capacitor, MOSFET gate leakage (a tiny amount of current), MOSFET drain to source "ON" resistance and gate threshold voltage, and reverse leakage through the 1N4001 diode. The best way to select component values is to start with the math but base final component choice upon tested parts you plan to use.

This circuit is "retriggerable", so repeated events sensed by the PIR within the timers "on" time will extend the time interval.

Note that large value capacitors can vary wildly from their specified value - often -80% / +100% ! They also tend to have large swings in value with large changes in temperature. So you'll want to test the actual part before using it.

Check this link for more info about capacitors than you'll ever want to know: http://www.execpc.com/~endlr/Ceramic/ceramic.html

Well.....Good Luck, I've tried this circuit on a breadboard so I know it works, but I've never used it in the field.

Archilochus

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Another one by "popular request" - I'd use 4538s or 4047s in place of the 555s.

This circuit is intended to control the "Start record"/"Stop record" button on certain video camera remote controls. It can also drive a solenoid to push the "Start/Stop" button on the video camera itself.