This is not a Club project as such but is presented for anyone who would like an Auto Sleep function for that battery operated project. The source of the circuit is not known but I am indebted to Roderick Wall, VK3YC, for sending it to me. It is very easy to construct, takes the place of a normal switch and will control around 400mA of supply current.
If the pushbutton switch is pressed, the circuit will turn on. A press while the circuit is operating, the circuit will turn off. Forget to push the switch and the circuit will automatically turn off after a pre-determined time.
The secret to understanding how the switch circuit works is in the fact that, when in use, there is the load circuit connected between S (circuit +) and battery - and in the fact that the specified FET (2N7000)is an enhancement mode type (always off and requires positive bias (G-S) to turn on) and not a depletion mode type (always on and requires negative bias (G-S) to turn off). Unlike depletion mode FETs, enhancement mode FETS do not have an inherent through connection D-S. Bias is needed to create it.
Let's assume negligible voltage drop across D - S when it's operating, a battery voltage of 9V, a required circuit voltage of 9V and voltages are with respect to the negative battery terminal. D = 9V (battery voltage) all the time. S is connected to the circuit to be controlled, and at the first button press will be close to 0.
ON Function - Push the button and COM goes to 9V as it is connected to the battery positive terminal, capacitor + terminal also goes to 9V.
Release the button and COM is connected to G. G then = 9V, S is still 0. The positive bias on G creates the 'conductive bridge' between D and S and the FET now conducts. S rises to close to 9V, remember we said we would ignore any voltage drop across the FET, COM and G will now go close to 18V as the negative end of the capacitor is connected to the Source and the capacitor was initially charged to 9V. Your circuit is now powered via the FET 'switch' which is in series with the battery positive supply.
Manual OFF Function - If the button is pressed while the circuit is operating, COM is connected to 9V and the charge on the capacitor is, once again, forced to 9V. This is virtually the same as the operating voltage on S. Result? Any remaining charge on the capacitor is effectively shorted out, forward bias is removed from the FET and the circuit stops.
Auto OFF Function - Once the circuit is running, the capacitor will slowly discharge through any available path. In this circuit, leakage paths are extremely small. Your circuit will remain powered as long as the FET is forward-baised. The capacitor discharges via any Gate, diode or internal leakage until it drops to near the Source voltage or the minimum FET forward-bias voltage at which point your circuit will start to turn off. Attach any measuring instrument to the capacitor and see how quick it stops. Under normal use, the circuit won't suddenly stop. It will just peter out and die - how quick depends on the attached circuit that you are powering.
Parts - There is nothing really special about any of the parts except for the FET which must be an enhancement mode type. A 2N7000 is specified here. The capacitor must be at least a 10V type and its value is related to your desired operating time. A value of 4.7uF allowed operation for about 7 minutes in a test circuit. The diode may be a 1N914 or similar. The switch is a changeover type with momentary contact.
Email: General Information: vk3cmz@marc.org.au.
Created 2009 by
Kevin Crockett. Comments and suggestions may be e-mailed to
