Touch-activated timer switch extends battery life
Israel Schleicher, Prescott Valley, AZ; Edited by Martin Rowe and Fran Granville - July 10, 2008
A certain type of cordless optical computer mouse operates on two AA alkaline cells. It has no power on/off switch. When not in use, it automatically reduces power consumption by switching its light source on and off at a low duty cycle. Nevertheless, this function unnecessarily drains the battery, and it is annoying to often find the device inoperable. The solution to the problem is to add a battery switch that automatically disconnects the battery after a preset time. This approach requires no disassembly or other kind of tampering. This Design Idea describes two distinct implementations of a touch-activated timer switch that you can add to many battery-operated gadgets that you might inadvertently leave on.
The circuit in Figure 1 illustrates an analog implementation of the switch. Figure 2 and Figure 3 show digital implementations. The idea is to insert a 30-mil-wide strip of dual-sided PCB (printed-circuit board) between the negative pole of the battery and the spring contact of the battery holder (Item A in the figures). Q3 is a low-threshold MOS transistor that connects between the two sides of the strip and serves as the switching element (Figure 1). C1 is a 0603 X7R ceramic-chip capacitor, and R1 is a 0603 chip resistor. You mount Q3 and all associated components near the upper edge of Item A. You insert a narrow strip of thin brass, Item B, in series with the positive pole of the second cell. You connect it to the circuit with a piece of thin, flexible wire. Touch contacts C and D comprise short strips of self-adhesive copper tape that you attach outside the battery compartment. Thin and flexible wires connect C and D to the circuit.
Q1, Q2, and C1 form a monostable flip-flop. When the switch is off, C1 does not charge, and both Q1 and Q2 are off. When you momentarily touch both C and D with bare fingers, current through your hand charges C1 to the threshold level of Q2. Both Q2 and Q1 turn on, discharging C1 through Q1 and your conductive fingers. The voltage level at the gate of Q2 is then close to the battery voltage. After you remove your fingers, the leakage through the internal gate protection of Q2—the zener diode in the figures—causes the voltage at the gate of Q2 to slowly drift lower until it reaches the threshold level of approximately 1.3V. Q2 exits conduction and, with Q1, causes a regenerative action to quickly turn off Q3.
The switch remains off until you again touch C and D. Item E is an optional contact similar to C and D. If you touch E and D, the switch turns off. Using a value of 0.01 µF for C1, you obtain a delay of approximately one hour. Because the gate leakage is on the order of a few picoamperes, you must clean the circuit with a flux solvent and then coat it with a drop of wax or epoxy resin.
In some cases, you might want to be able to adjust the timing of the switch. The circuit in Figure 2 provides that option. It uses a tiny microcontroller in an SOT-23 package. Listing 1 contains the touch-activated timer switch. Items A, B, C, and D are the same as those in Figure 1. When the switch is off, the PIC10F200T microcontroller is in sleep mode and consumes practically no power. When you simultaneously touch contacts C and D, the level at Pin 1 of IC1 goes high, and the microcontroller starts to tally the time that Pin 1 remains high. After 0.5 sec, the buzzer sounds a short beep. The buzzer then sounds two, three, and four fast beeps in 0.5-sec intervals. By immediately releasing contacts C and D after hearing any number of beeps, you can set the switch for 30 seconds, 30 minutes, four hours, and eight hours of operation, respectively. The choices of operating times are arbitrary; you can modify the code in Listing 1 to whatever fits your application. Jumper switch J1 is optional. If you leave it open, touching C and D turns it off. Short-circuiting J1 disables this option, and the switch will turn off only at the end of the programmed time. As is the case with the analog implementation, you mount all components except the buzzer at the edge of Item A. The buzzer is a small piezoelectric element with a resonant frequency of 4 kHz and can easily fit inside the battery compartment.
In some cases, you may not have access to the negative contact of the battery holder. The circuit in Figure 3 addresses this situation. It is essentially the same as the circuit in Figure 2, except that you place Item A in series with the positive pole and attach B to the negative pole of the battery. A P-channel MOS transistor acts as a switch, and you modify the microcontroller’s program to provide a low level to drive Q1. A comment in Listing 1 indicates the proper line of code for the options in either Figure 2 or Figure 3.