Timer circuits

Timer circuits typically use a 555 IC. A much simpler circuit only uses 3 components. Such a circuit can be used to switch off a LED after a certain time, typically between 1 and 15 minutes. The exact timing of the circuit isn’t precise and depends on a RC network.

The circuit uses a P-Channel or N-Channel MOSFET. Ideally, this is a logical level MOSFET. In the datasheets, these MOSFETs typically have a low gate threshold voltage (\(V_{GS(th)}\)) (ideally around 1V for N-Channel and -1V for P-Channel). These MOSFETs are typically completely on at \(V_{GS}\) = 10V / -10V, logical level MOSFETs will also have a value at 5V / -5V. In the datasheet, you will find that value under \(R_{DS(on)}\), the static drain-source-on resistance. You don’t need a power MOSFET (TO-92 package is OK), as battery operating devices don’t use a lot of current. non-logic level MOSFETs are only OK for higher voltages, but as this circuit depends on a low treshold, logical level MOSFETs are preferable.

This circuit only works with MOSFETs, as these transistors are voltage controlled and have a very high resistance at the gate, so the capacitor is only drained via the resistor and not via the gate. A BJT transistor won’t work, as these are current controlled and would drain the capacitor during operation (a large resistor won’t work here, because this would also imply a small current, which would limit the amount of current that can pass through the BJT transistor).

N-Channel MOSFET

You will use a N-Channel MOSFET for low-side switching (switching the connection to ground). To turn the MOSFET on, you will have to apply a positive voltage to the gate of the MOSFET. The circuit works as follows:

• A small burst of current is applied to the gate of the MOSFET, at \(V_{cc}\) (for example 5V).
• This will fully charge the capacitor (this is OK with batteries, as the resistance of the batteries will limit the current. A small resistor might be necessary when there’s no current limit);
• The voltage at the gate of the MOSFET is now \(V_{cc}\), above the treshold, so the MOSFET will turn on;
• The capacitor will slowly lose its charge via a large resistor to ground. The voltage at the gate will drop slowly;
• At a certain time, the voltage at the gate will reach the treshold of the transistor, turning the transistor off.

P-Channel MOSFET

You will use a P-Channel MOSFET for high-side switching (switching the connection to positive power). To turn the MOSFET on, you will have to apply a negative voltage to the gate of the MOSFET, with respect to the gate (this means: connecting the gate to ground). The circuit works as follows:

• The gate of the MOSFET is connected to ground for a short period.
• This will full discharge the capacitor (this is OK with batteries, as the resistance of the batteries will limit the current. A small resistor might be necessary when there’s no current limit);
• The voltage at the gate of the MOSFET is now 0V (= -\(V_{cc}\) with respect to the source), below the treshold, so the MOSFET will turn on;
• The capacitor will slowly gain charge via a large resistor to \(V_{cc}\). The voltage at the gate increase slowly;
• At a certain time, the voltage at the gate will reach the treshold of the transistor, turning the transistor off.