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Electronic lock with a window comparator

The circuit shown here is a window comparator connected as a resistance detector. As will be seen later, it is well suited to the construction of electronic locks.

A window comparator only responds to DC voltages in a defined limited range. If one omit the resistor Rref, the circuit would work that way. In the dimensioning shown, it would react to a tightly tolerated input voltage if this corresponds to half the operating voltage. Since the presented circuit provides an inverted output signal, the lamp would go out at an input voltage of about 6 volts, on the other hand, shine lights. The circuit could serve as a voltage monitor, for example, which triggers an alarm sound at too large a deviation from the target voltage.

Due to the resistor Rref, only half the operating voltage is present at the input when the externally connected resistor Rx has the same resistance value. Rt determines the tolerance, i.e. the range of resistance values to which the circuit responds. At a value of 1.5 kOhm, it is less than 15%.

The circuit is therefore very well suited as an electronic lock, which can only be opened with the right resistance. For example, in an practice room where different bands were playing, should to prevent the unauthorized use of our amp equipment. For this purpose, the circuit was installed in the devices to be protected. A jack not needed in the respective device was connected to the input of this circuit. A miniature relay was connected instead of the small lamp. An NC contact then interrupted the absolutely necessary audio connection from the preamp to the power amp. Outwardly, the amplifier then made the impression of a defective device. However, a device re-equipped in this way could be brought to life in seconds with a jack plug in which a 33kOhm resistor was soldered in place. Not everyone has such a plug in their pocket!

However, if the operation of this electronic lock is known, it can quickly be outsmarted with a jack plug with soldered potentiometer. For greater safety, the circuit can be set up several times. The outputs are simply connected in parallel because a connected relay will only drops down if all the resistors connected to the inputs match the reference values. You should use different values for the individual resistors, which can then be soldered, for example, in a multi-pin connector (e.g. a DIN-plug). By additional triggering a timer circuit with a further transistor output, it is possible to limit the amount of time in which different values can be tried out to trigger the circuit. The electronic lock can then hardly be cracked without intervention inside the device.

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