2008-03-29
RF Remote Control Experiments
Some time ago I tacked some LM567 Tone Decoder chips onto a Futurlec order as almost an after thought. I've always wanted to play with them, but it wasn't until Friday night that an immediate use jumped into my head. Like so many of my experiments, the sudden realisation of what I could achieve with them totally derailed the other work in the experimental pipe line, and re-purposed it towards this.
I did give me an excuse to finish building the quench filter and AF pre-amplifier module, as is was needed for these experiments. It comprises of two cascaded Sallen-Key low-pass filters set for about 3.2 kHz and a common emitter pre-amp.
The TX and RX modules from the previous 11 metre AM transceiver work were a perfect test bed for the the tone-coded RC prototype. Basically I lashed up the TX with my signal generator as an AF source and the RX with some filtering to feed the LM567s. Quickly enough I had a LED lighting up when I keyed the transmitter.
The LM567 can sink about 100 mA, so quite reasonable loads can be switched by it. Unfortunately its saturation voltage is fairly high, but it can directly switch small motors with some snubbers to protect it. A 5.9 Volt motor from a CD player I recently ratted was fitted with an eccentric weight (an aligator clip on the shaft) and rapidly provided remote controlled vibration amusement. The most obvious application is some kind of RC robot, but before I got ahead of myself to achieve that I needed at least two (and better four) channels, and would eventually need to work out a H-bridge to drive more non-trivial motors with some logic to prevent glitches shorting the rails, etc...
A pair of LM567s were wired up on the protoboard and multi-turn trimmers used to align them to separate, but closely spaced tone frequencies. The signal generator swept through their lock ranges and operation confirmed.
I still needed to prove the circuit would work with the two channels concurrently, but to do that I needed to build two AF tone sources. A pair of phase-shift oscillators where built, and after some number-crunching and buffering hell I had a suitable two-tone signal source for modulating my TX. This is basically frequency division multiplexing. Having proved the RF system at this point I dispensed with it for the remainder of the tone generation and decode work, favouring direct connection through a suitable attenuator while stabilising the design.
Now quite happy that two concurrent channels would work (not too surprising as DTMF decoding is an application hinted at in the datasheet - hopefully I can do four with still sufficient IMD/HD and noise margin), it is simply a matter of building the circuit in a more permanent form and tackling the (typically harder for me) task of the mechanical side of a robot of some description.
When I've got all this nutted out I'll post the circuit diagrams of both the voice walkie-talkie and the remote control (and whatever it ends up controlling).
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