serrure a combinaison optique

This interesting circuit appeared as `Circuit of the day` in the Siemens Components Report no. 2/78. The circuit is easy to construct and calibrate, it will operate over a wide range of supply voltages (8 ... 20 V), and it is quite accurate. It is also ideally suitable for use in cars, since it is insensitive to temperature variations over a wide range and has high noise immunity. Figure 1 shows the complete circuit, using a 1 mA instrument (a modification for more ro-bust 10 mA instruments will be discussed later). With the switch in position 1 the circuit operates as a rev. counter. The input (CB) is connected to the contact breaker; R2, 03 and C2 provide protection against high voltage transients. When the contact breaker opens, the voltage at the input rises; as soon as the voltage at pin 6 exceeds the trigger voltage at pin 8 (i.e. 3 V, derived from the reference output, pin 5), the open-collector output at pin 13 is turned on for a period set by R3 and C5. During this set period, current therefore flows through the 1 mA instrument. This current is made virtually independent of supply voltage fluctuations by the simple expedient of deriving it from a second refer-ence output, pin 10. The mechanical inertia of the meter is sufficient to integrate the current pulses, resulting in the desired RPM indication.