A Very Simple Remote Control

Around 1985, a British firm called Holtek started making integrated circuits that could transmit and receive simple addresses.

You could code the encoder chip by taking anything between none and all eight address lines to logic 0 or leaving them internally pulled up to 1.

That gives 256 combinations, because an 8 bit number is 256 (2^8 = 256)

It also had 4 "output" pins, so if you took one of them low, an equivalent pin on a connected decoder went high.

So, the extra bits gave 4096 combinations, and if the two chips were interconnected by radio waves, you had a remote control lock!

They're called 2^12 encoders and decoders, because 2^12 = 4096.

Today, Holtek is Taiwanese, and makes very little mention of its start and when it actually started. I can only remember the approximate year because I remember what I was doing when a rep asked me if I could make use of them.

There was also an encoder with an infra-red LED driver, which would have been excellent for this project, but Holtek discontinued it.

So, if we want to use the encoder meant to work with a radio transmitter, we'll have to supply our own infra-red LED driver.

The I.C. set is possibly only intended for garage door locks today, and perhaps it isn't a great idea seeing there are only 4096 combinations! But that's no reason for Holtek to stop making them (just in case they're thinking of doing so).

The chips also have oscillators to clock the combinations, and I guess by spacing clock frequencies apart - maybe 4 of them - it could be possible to get the number of combinations to 2^16 or 65,536.

We will see shortly how we can get them to communicate to produce a simple motorised potentiometer remote control to turn the volume up or down.

But we'll also need some logic to drive the motor forward, backwards or stopped.

Logic

The HT12D decoder latches to the encoder's last transmission, so say we have two states (that's two output pins on the HT12D), one for volume up, the other for volume down.

But what will happen is when we stop pushing the button on the remote control, the volume control will still be turning all the way to one extreme or the other, due to the latching action.

However, the HT12D has a pin indicating transmission in progress, and that doesn't latch, so reverts to its resting logic level once the key press has stopped.

What we need is a motor driver which uses the up or down latched data along with an enable signal.

So if latched to down, the transmission in progress enables down, and if latched to up, the transmission in progress enables up.

Once the key press stops it is no longer enabled and the motor stops.

There just happens to be a chip called a half-H driver which has these exact logic inputs.

We need two half-H drivers which means one half causes the motor to turn one way, and the other half the other way. These are interlocked such that if both inputs are high, or low, which will never happen, the motor gets the same voltage at either end, which means it won't run.

Only when one is high and the other low, will it turn in the wanted direction.

But nothing will happen at all until it gets an enable signal, and that will be from the transmission in progress output.

Great!

These I.C.s are actually quadruple half-H drivers, so we only need use half of one. But the pins are arranged such that one side can be paralleled with the other, and that means it can drive twice the current!

The two available I.C.s go by different names: one does 0.6 amp and the other does 1 amp.

What current do we want? The motor potentiometer max. current is 150mA, and that's with its clutch slipping at its end stop. Normally it's around 100mA.

If we parallel up the lower power I.C., it can do 1.2 amps, and as we only need 150mA max., it's no stress. The average load is about 1/10th capacity, and that means the I.C. won't run hot, and we shouldn't need any heatsink on it.

Here's the chip names: SN754410 (0.6A) or L293 (1A). We can use either.

So now, we just need the transmission medium.

Infra-Red Transmission And Reception

Remote controls use invisible light as their transmission medium. It's called infra-red. If not, then any light pointing at them could falsley trigger them. Even so, there is another source of infra-red - the sun - which could also falsley trigger them.

The answer is to use pulsed infra-red, and so remote control receivers compare the transmission frequency with what it's supposed to be, and only then give an output.

The most common frequency given over to infra-red remote control is 38kHz. It's a frequency higher than the audio spectrum and lower than other frequencies used in equipment commonly operated by remote controls.

For the remote control to work adequately, the transmission pulses aimed at the remote control receiver must be very close to 38kHz or the receiver might not operate each time.

The Vishay TSOP4838 38kHz IR receiver is suitable.

No Clocks!

The TSOP4838 IR receiver detects 38kHz through use of a bandpass filter, and doesn't use a clock as a comparator, hence there isn't any need for a clocking circuit (oscillator).

The HT12D decoder oscillator is disabled in standby mode, and only activated when a logic 1 is applied to its data input, which is when the TSOP4838 IR receiver is receiving the IR transmission.

I'd say this remote control receiver is ideal for incorporation into a high fidelity audio device, because, except for the reception of a control signal, it cannot disturb.

Receiver Circuit

The TSOP4838 IR receiver outputs low (logic 0) on reception of a signal, and the HT12D requires a high (logic 1) on its data input pin to receive, so that requires an inverter. As we have no logic chips in this receiver circuit to borrow an inverter from, a small signal transistor arranged to switch shall do.

Notes On The Receiver Circuit

Voltage Regulator

All components shown either must operate on +5VDC or will operate on +5VDC, so the obvious choice is a +5VDC supply. Fine!

However, we don't know the voltage available within the product (preamp, integrated amp, receiver or whatever) and here we have to be careful.

The 7805 voltage regulator is specified for up to a 35 volt DC supply, after which its sustaining voltage is exceeded and it will probably blow its insides out through the plastic encapsulation. Also note that the supply voltage within the application might not be regulated so you need to allow a safety margin.

A 35V unregulated supply could easily rise to say 42V for numerous reasons. A 6V8 zener diode placed in series with the 7805 input affords some protection, but as pot motors can demand 150mA, then the 6V8 zener will be dissipating over a watt. A 1W3 zener would soon expire, but a 5W zener should survive, although it will get hot, and it should have PCB pads with plenty of copper fill surrounding them to act as a heatsink. Even with a supply of 30V I think the zener a good idea.

Where a 5W zener isn't available an "amplified zener" comprising a suitable pass transistor, low wattage zener from collector to base, and a 91 ohms base-emitter resistor may achieve the same, but with an overall 7V5 voltage drop.

Also, consider the 7805 power dissipation: the voltage drop input to output is the supply voltage minus 5 volts, so if the voltage at its input is 35V then with 150mA motor current, the 7805 will dissipate 4.5 watts, and it cannot do it on its own - it needs a heatsink to limit the temperature rise. A 5 to 6 ºC/W heatsink should be considered the minimum to keep the junction temperature safe.

If the preamp has a supply voltage of 20 - 24 VDC, the voltage regulator input can be directly connected, but a heatsink of no greater than 10ºC/W is required.

The 100nF capacitor across its input should suffice within 100mm (4 ins) of the main power supply, but much further and it should be as large as 1uF.

The 2.2 ohm resistor in series with the 100uF capacitor on the regulator output is necessary to prevent it trying to oscillate as the pot. motor load runs and stops.

The best place for the 100nF decoupler (C4) is close to IC1 pins 18 and 9.