Automatic Switch

034 GENERAL INTEREST AUTOMATIC SWITCH Many battery-operated instruments, such as digital voltmeters, have a change-over on-off switch. The automatic switch presented here makes full use of that. When the instrument is off, Cl charges fairly quickly via R2 and Dl. As soon as the instrument is switched on, Cl discharges slowly via R1. As long as the dis- charge current exceeds a certain level, T1 is switched on by the voltage across R1 and the supply to the instrument is on. When, after a few minutes, Cl is almost completely discharged, T1 tog- gles and the supply to the instrument is switched off. The period between switch-on and switch-off may, of course, be varied by different values of Cl. (Ph. Bosma)