automatic switch off

elektor july/august 1985 11g 1111104111041111 If you are one of the many who fre- quently forget to switch off their digital multimeter, this circuit is for you. When this little circuit, which is in- tended to be incorporated in the multimeter, is switched on, capacitor C1is connected to the +9 V line via D1. Since C, is discharged, the gate of T3is also at +9 V which causes T3 and T2to conduct. The meter is then switched on. automatic switch off Capacitor C1slowly charges via R2. After about 2 or 3 minutes, the poten- tial at the gate of T3becomes too low to keep the FET in conduction. Tran- sistor 12then also switches off, and the battery is disconnected from the multimeter. " Transistor T, ensures that when the BS250 BS 170 D/G multimeter is switched off manually, capacitor C1is discharged. When the multimeter on/off switch, Si, is opened, a base current will flow to the negative terminal of C1via Ri and R2. Transistor T1then conducts and discharges C. The circuit is thus imme...