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Friday, 27 May 2011

Mains Remote Control Decoder

This receiver/decoder forms part of a simple mains network remote control system, which also includes the ‘Mains Remote Transmitter’ and the ‘Mains Remote Encoder’. The decoder is built around IC1, which is a Holtek type HT12D or HT12F. For the receiver we use the same circuit as in the ‘mains remote switch’, namely a passive circuit tuned to approximately 143 kHz, since we assume that the transmitter is powerful enough to provide an adequate signal. Two 4069U inverters (IC2) are used to convert the received signal to TTL levels. D1 and D2 provide extra protection against noise pulses and the like. The sensitivity can be adjusted using P3, but you should bear in mind that over-driving IC2 can cause corruption of the data.

Mains Remote Control Decoder circuit diagram schematic projectThe trick with IC2 is that a small offset applied to the ?rst buffer causes the second buffer to be displaced from the middle (which can be checked using a multimeter), so that the following monostable multi-vibrator (IC3, a 4538) receives a usable burst as a trigger signal. IC3a is re-triggerable, which means that if a trigger pulse arrives within the set time, the output pulse is extended. However, if the set pulse width is too long, the output pulses are extended so much that the decoder will not recognize them as valid data.

Mains Remote Control Decoder circuit diagram schematic projectIC3a thus recovers the originally sent code. P2 is added to the circuit to allow the pulse length to be adjusted as accurately as possible, but an oscilloscope is required for this. In practice, the adjustment is not all that critical and P2 can be simply set to its mid-range position. The output of IC3a is fed to the decoder (IC1), which compares the recovered code with the settings of S1 and S2. If the received code matches these settings, output VT goes High and some sort of application can be energized via buffer T2. If you have in mind connecting an active buzzer to the buffer output, you must thoroughly decouple it using a 10-mH coil in series and a 100-µF/16-V electrolytic capacitor in parallel, since these buzzers can be a source of stubborn interference.

Mains Remote Control Decoder circuit diagram schematic projectThe second monostable (IC3b) is used to generate a supplementary pulse with a duration of roughly one second. The pulse length can be modified (by changing R2 and/or C2) to meet the needs of a particular application that requires a certain minimum duration. T1 acts as a simple buffer for this output. As already noted, in principle two different types of decoder can be used: HT12D or HT12F. The HT12D has four data-bit outputs (AD8–AD11), with the data being made available on the SIL header K1. In this case it is better not to fit S2. If an HT12F is used for the decoder, K1 has no function, but a 12bit address can be set.

Mains Remote Control Decoder circuit diagram schematic projectNaturally, the oscillator of the decoder should be tuned to match the encoder used with the transmitter. For the HT12D/F, the oscillator frequency is 50 times that of the encoder. That means that here the oscillator must be set to around 112 kHz. According to the related curve on the data sheet, this requires an external resistance of approximately 115 k? to be connected between the OSC1 and OSC2 pins. This can be precisely set using P1, and the potentiometer also allows for adjustments to compensate for various tolerances.

The power supply for the circuit is designed according to the usual standard con?guration, with the transformer (Tr2) being intentionally somewhat over-dimensioned to provide extra capacity for powering small applications (buzzer, LED etc.). Building the circuit is a simple task if the illustrated printed circuit board is used. Since the power supply (including the transformer) is fitted on the circuit board, the amount of wiring required is minimal.

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