This circuit uses just two standard logic ICs which, with the accompanying receiver, forms a four-channel remote control and has been designed for use with the ‘Audio/Video Switch’ and the ‘Switchbox for Loudspeakers’ (shown elsewhere in this issue, as is the IR Receiver).
Each pushbutton is connected to an input of a shift register (4021), which is clocked by a binary counter/oscillator (74HC4060). A cycle is started via one of the diodes connected to each pushbutton and a differentiator-network (C1/R4), creating a short pulse that is fed to the shift register and counter/oscillator. The parallel-load input at the shift register then becomes momentarily active, causing the shift register to latch the data at the parallel inputs. At the same time, since the pulse is very narrow, one of the four pushbutton inputs will be high (in spite of any longer pulses that may be caused by switch bounce). The pulse also goes to the reset input of the 4060 and starts the oscillator.
The oscillator around P1/R2/R3/C2 is set to about 36 kHz because IR receiver modules are widely available for this frequency. Pin 7 is the Q3-output of the 4060 and clocks the shift register. The data at the parallel input is now output in a serial format at QH (pin 3). When QH is high the emitter of T2 is made high via T1, and a pulsed current at 36 kHz will flow through LED D6 via R5 and T2. Pin 1 (MSB) of the 4021 is permanently high and is clocked first to the output. This bit functions as the start-bit for the receiver. Since the receiver clocks the data on the rising edge, the start-bit has a length of only 8 cycles at 36 kHz. The rest of the data is modulated at 16 cycles per bit. Output Q7 (pin 14) of the 4060 is connected to the oscillator via a diode, which causes the oscillator to stop after 8 clock cycles of the Q3 output (pin 7) and the circuit becomes idle. The relevant code is therefore sent only once for every button press. A new code will only be transmitted when a pushbutton is pressed again. The only current drawn now are various leakage currents and the current through D5/R2/P1/R3 which is about 10 µA. So even without an on/off switch the batteries should last for years (assuming a capacity of 750 mAh for AAA cells).
P1 has been added to compensate for the various component tolerances and to tune the transmitter and receiver to each other. JP1, JP2 and JP3 are a bonus and can be used for addressing purposes or for a possible expansion with more pushbuttons. At the receiver side these bits are made available on three outputs. The PCB has been designed to fit inside a plastic box with an integral battery compartment (see parts list). If required, the PCB can be made a bit smaller by cutting off the blank areas (where the mounting holes are). That makes it possible to use a smaller enclosure, especially if a 3 V lithium cell is used for the supply.