Magic Key

idea: Elex Team

An infra-red remote control for an (electromagnetically operated) lock? Nothing special, you will think. But some umpteen years ago this looked like magic, and it was very satisfying to build something like that yourself. Even if it was only to amaze the neighbours. Here you can see how it was achieved, using only a small number of components.

First we need a transmitter, which produces some kind of IR signal. To keep things uncluttered, a simple 555 oscillator is used which produces a rectangular wave at a frequency of 3.5 kHz having a duty cycle of 75% (the output is high 75% of the time and low 25% of the time) – see Figure 1.

Figure 1.

That much about the transmitter; now the receiver. A photo diode type BP104 is used for the IR detector (see Figure 2), which converts the received light pulses into an electrical signal that is then amplified by transistor T1.

Figure 2.

The alert reader will have noted that T1 operates practically without any quiescent current, so that the circuit is relatively insensitive: the transmitter has to be held quite close to the detector.

The remainder of the signal processing is taken care of by IC2: this is a tyope 567 PLL tone decoder that in a relatively small frequency range (in this case about 3.5 kHz) decides whether a signal is received or not.

Why a tone decoder? Simply put. because otherwise we would need a discretely built, narrow-band bandpass-filter, and that is not so easy to make. Here a PLL circuit ensures that only ‘valid’ signals can open the lock (or whatever you decide to control with the relay contacts).

By the way, about the relay: this is controlled directly from the IC, an additional transistor driver is not necessary (since this is already built into the IC). Re1 has an internal back-emf diode.

And a final remark: the potentiometer P1 in the transmitter is used to adjust the frequency of the IR signal so that the receiver reacts correctly.