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Tiny 1.2V FM receiver for decoding 144MHz DTMF

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neazoi

neazoi

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Hello, I would like to control a device, using DTMF commands that are modulated in FM at an RF carrier of 144MHz.
So I need a suitable receiver that can demodulate 144MHz FM into DTMF tones. Then I will handle the DTMF tones to switch things on the device.

What is the simplest design I can build?

The receiver will only be connected to the RF input through a coaxial cable, so no good sensitivity or selectivity is demanded. 1.2V is only available to power the receiver circuit.

Any ideas?
 

Years ago, Philips made the TDA7000 then replaced it with the TDA7088 that works from a supply as low as 1.8V. You can use the simple voltage stepup circuit in a Joule thief or solar garden light to stepup the 1.2V. The IC is not available from Philips anymore but a Chinese company copies it and a radio with it is available at The Dollar Store costing only $1.00 with battery. I got two of them for free by buying cheap things in the store.

The radios are awful for sound but might be good enough for DTMF.
 
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    neazoi

    neazoi

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Audioguru said:
Years ago, Philips made the TDA7000 then replaced it with the TDA7088 that works from a supply as low as 1.8V. You can use the simple voltage stepup circuit in a Joule thief or solar garden light to stepup the 1.2V. The IC is not available from Philips anymore but a Chinese company copies it and a radio with it is available at The Dollar Store costing only $1.00 with battery. I got two of them for free by buying cheap things in the store.

The radios are awful for sound but might be good enough for DTMF.
Click to expand...

Thanks!
I was thinking of a simple transistor circuit, or even a passive diode discriminator, since the input signal is high (>50mW). This would require no power at all for FM receiving.
 

I don't have a design to hand but pocket pagers use 153MHz and will run from a single AAA battery. They use superregenerative receivers.
As a cable is being used, is there a reason why the DTMF can't be sent as plain audio tones? (no RF)

Brian.
 

betwixt said:
I don't have a design to hand but pocket pagers use 153MHz and will run from a single AAA battery. They use superregenerative receivers.
As a cable is being used, is there a reason why the DTMF can't be sent as plain audio tones? (no RF)

Brian.
Click to expand...

Yes, in fact the transceiver is a handheld and I need to control a device (preamplifier and other circuitcity) that is connected only to it's antenna, so that the radio is not modified.

One way without RF could be to have an mike amplifier on the device and respond to DTMF audio tones that are commung out of the transceiver speaker.
 

DTMF detection depends on critical similar amplitudes of the two frequencies (I think it is called twist) so that other sounds will not cause false detection. A speaker in a handheld transceiver will probably have audio frequency peaks and notches that will cause unreliable DTMF detection.
 

'Twist' is the correct term and telecoms systems are designed so the line level of tone groups are within (typically) 2dB of each other but most DTMF decoders are very tolerant of the levels they can accept.

The bigger problem is finding a decoder that will work from 1.2V, it is possible to do it in software on procesors that work to just below 2V supply but I don't know of any '8870' type decoders that work below standard TTL supply voltages.

Brian.
 

Is the DTMF decoder's radio used on the transmitter's antenna? Then a crystal "radio" is all that is needed. FM can be slope-detected like a regen radio.
The voltage boost circuit in many cheap solar garden lights uses an IC and it works fine from a single 1.2V rechargeable Ni-MH cell until its voltage drops to about 0.8V.
 

The problem with slope demodulation is the deviation is low (~5KHz) so at 144MHz the Q of the tuned circuit would have to be very high and it's frequency very stable. To make matters more complicated, there may be other stations using frequencies only a few KHz away.

A simple down-mix with a crystal oscillator to bring 144MHz down to < 1MHz would make signal recovery much easier and would be less complicated and allow tight selectivity with more manageable tuned circuits. If zero-IF could be used it would give even better results.

Brian.
 

A regen and super-regen radio detects AM, not FM. It can use slope detection to detect FM and some very cheap FM radios do it like that. The FM broadcast band is full in many cities.
 

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