infra-red controlled remote switch

Nowadays, several manufacturers produce ICs that combine an infra-red receive LED with associated amplifier and demodulator. The type SFH505A from Simens used in present circuit has, in addition, a band-pass filter to minimize any interface. It is best used in conjunction with an infrared transmitter in the next article.

The output of IC₁  is limited to a maximum pulse width. This is used to advantage by the associated transmitter to span as long a distance as possible. Most commercial remote controls use modulated data transfer in which the pulse width is smaller than IC₁ can handle. The difference is easily detected by an integrator. R₂-C₂ and a Schmitt trigger formed by -T input of monostable IC_2a.

The time constant is just short enough to ensure that at maximum pulse width of IC₁ the voltage across  C₂ is just under the trigger threshold of  IC_2a (IC₁ has an active low output which is the way it is connected to the negative trigger input of IC_2a ). Note that remote control units for, Sony or Philips, have no effect on the present circuit.

The monostable is retriggerable so that any bonus at the transmitter does not affect the wanted state of the circuit. The monotone has been set at about half a second. The circuit cannot, therefore, switch on and off rapidly. If, nevertheless, several or repeated pulses at the trigger input, they cause lengthening of the output pulse, but the state of circuit gets changed only once.

The output at pin 7 of IC_2a clocks D-type bistable IC_3aat the last trailing edge. The bistable is arranged as a binary scaler so that circuit can be switched on and off by repeated transmission.

Diode D₂ indicate that IC_2a has received a trigger pulse. If the interface is experienced from other remote controls, this indicated by D_2.

The output of IC_3a energizes one of the relays via T_1. At the same time, it triggers monostable IC_2b which energizes the second relay via IC_3b and T_2. In this way, the second relay is energized a good half a second after the first. This enables several apparatus to be switched on in two steps. Diode D₃  and D₄ indicate when the relays are actuated. This relays can switch up to         2000 VA.

The switch-on current of an equipment may be limited by first switching on the mains voltage with one relay via a series resistor and use the second relay to short out the series resistor.

The circuit draws a current of about 0.6mA when relays are unenergized and LEDs are out. When the relays are actuated and LEDs light this increases to 125mA.