Circuits Galore By Jos Hahn


There is obviously a lot of interest in telemetry so this month we'll look at DTMF decoders. Of course, firstly you need a tone encoder, but, they are available ready made, like DTMF mikes and small handheld DTMF pads, complete with a small loudspeaker to hold in front of the telephone mouth piece. Nowadays, everything is done with DTMF tones. If you want this, press one, if you want that press two etc.DTMF Decoders

Please understand that I am an engineer and not a lawyer, if you convert a radio to a frequency for which it was not designed, it must be for receiving purposes only, this is the way the Americans do it. I am extremely good friends with the ACA and I would like to keep it that way. We radio people will obviously build them ourselves.

If you are a farmer, who would like to switch on a pump many miles away, you will prefer to push one knob to transmit a DTMF code to instruct the pump to do its thing and, it would even be nicer, if the pump would answer back with "No worries mate, you've turned me on".

This is called a handshake and it's same story for the switch-off cycle.

The easiest way to encode and decode would be 1 (one) digit only, but that is rather risky. Let me explain this a bit further. You might be under the impression that this globe is populated by homo sapiens only, but you're wrong. There are plenty of aliens from the planet Lunicia out of the solar system called Imbicilia Maximus and they are able to copy your tone and they will keep turning your pump on and off and amuses their small minds.

Therefore, the more digits, the safer. There are a few more ways to kill this cat, such as using different tone frequencies by changing the reference xtal. Normally, your fingers are doing the dialling, inclusive of including small pauses during the digits. The automatic dialler has to do the same. If you look at last month's two tone DTMF roger beep, you can see that I included a pause between the two tones. I have designed a lot of burglar alarm systems in the past of which many are still commercially built and marketed so experience does count for something.

In the next Circuits Galore I will include an universal four digit DTMF encoder, which you can use for a hell of a lot of purposes. This time I did make a pc board for it -which I don't usually do.

If you would like to make a keypad encoder, first make sure that every button is a make-contact, if you would like to use the 5089.

The wiring is shown in Fig 1.

I once made an enormous, big keypad for an 90 year old bloke, 40 x 40 cm with huge push buttons....! knew you would be interested to hear that In my circuit all switches are grounded and we need 16x2 diodes to complete the encoder. If you would like to see a light every time you push a knob no problems, that is all on the circuit!

If you want to hear the tones, just add a LM386, two caps and a loudspeaker. Just look in the Dick Smith catalogue. The loudspeaker version can be used for direct radio modulation via the mike. You have to use one hand to PTT the mike and the other to push the keypad switches.

"When will de-bloody-decoders get a mention" some will ask, answer is simple: Now.

DTMF DecodersWe'll start with a decoder with two 7 segment displays. This will show the two digits of the ID roger beep of last months CG. It can also be used for any other two digit encoders of course. There are a few segment display driver ICs available, like the 4511 and 4543. The second one is advertised in some catalogues for LCD only, but that is a lot of bull, it handles both common anode and cathode displays and it has a positive latch. So, let's forget the others and go for the 4543 from now on.

It also makes a nicer 6 and 9 than the 4511. The brightness of the display depends solely on the quality of the displays, so buy Hi-Bri ones, if that is what you like.

If I only have two DTMF digits, it is relatively easy to get the displays to latch, if I have a lot of digits, we will do it differently, as you will see in the future. The circuit is diagram Fig 2.


IC9 is our regular DTMF decoder as described in last months CG. The STD and TOE are connected, this means that the four outputs are only showing during the receiving of the valid DTMF tones.This circuit needs 5V, so a regulator Tr4 is included. Let;s say we receive 4 and then 8 as an example, firstly 4, STD and Q3 are high (5 volt) during the tone, both 4543s have Pin2, B high. ICS will get a positive pulse on its LD input (through C21 R27) and latches, display 2 will show 4. At the same time the STD voltage went through D10 to C18 and R5, so input 1 of IC7 is high as well, output 2 and input 3 are low and output 4 is high. IC6 also gets a positive pulse on its latch input (through C19-R22). Display 1 will therefore also show 4. Digit 4 goes away and STD goes low. After a short pause digit 8 will be received. STD is high again, as is Q4 and input D (Pin4 of IC6 -ICS). ICS will latch again and display 2 shows 8, output 4 of IC7 is, however, still high through the charge of C18 and IC6 won't change. This display now shows 48. With this system we are limited to digits 1-9, everything higher than ten will be too much for the 4543 and it will be blank. It is possible to add extra 4543's and displays to show also digit 0, #, *, A, B, C, but that is another story.

Till next month   Jos

To Circuits Galore Pt 2

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