Circuits Galore By Jos Hahn

Jos Hahn of Syncro Electronics continues his series of circuits galore

What do you want to do with a DTMF encoder and decoder?

We continue this month with the DTMF en/decoders using the 4028 and 4514. The 4028 has its limits, as it only has 10 outputs, but with the assistance of a few inverters, we can extend this to the full 16.

We also have to look at the desired application. What do you want to do with a DTMF encoder and decoder? ID code, selective calling, switch repeaters on and off, pumps, lights, sirens, portable traffic lights, water level of a dam, temperature-measuring from a distance, burglar alarm, boat watch, etc.

We only have to convert our information into DTMF tones and transmit it. If you only need something switched on for a short time, you simply charge an electrolytic cap together with the high impedance of a CMOS gate. I have used a relay a few times, but that is for heavy duty tasks, like pumps, motors etc. For pure electronics, I use 555s, nand gates and inverters.

Back to a decoder using the 40287, still using a relay (FIG 1 ). Let's take tone 9 to switch on and 1 for off. The output of tone 9 is Q10 (pin 5) of the 4028. A high on Q10 will make pin 2 and 3 (IC3) low, so pin 4 goes high and switches the relay on via TR1. The voltage on pin 4 will keep pin 1 high as well, through D3 and R6. We don't need tone 9 anymore. By receiving tone 1, (Q2) we are able to make pin 3 of IC3 high and switch off the relay. If we want to use tones higher than 9, we have to add a few inverters, but let's forget the 4028 and go for the 4514 - don't forget we have plenty of them.DTFM Decoders and Encoders

We only have to convert our information into DTMF tones and transmit it. if you only need something switched on for a short time, you simply charge an electrolytic cap together with the high impedance of a CMOS gate.

With the 4514, we don't need extra networks, every DTMF tone comes nicely out of this 1C (FIG2). Beautiful. D is binary 0, so we have to give this output a helping hand with R5 and Dl. You must remember, that if we connect STD (strobe) with TOE, there will only be an output during valid DTMF tones. If you receive digit 8, output 8 of the 4515 will only be high during that tone. I hope, that everybody understands the DTMF encoder and decoders so far. No Questions? OK, let's go for a few more digits, it is a lot safer. I have three different ways to decode, I will share two methods with you, and the third one still makes a bit of money for me somewhere on our globe.DTMF Decoders and Encoders

We'll start with two digits. Note that they always work together with the circuit of FIG 2. If I say output 5, I mean output 5 of the 4514, as written inside the 1C block.

Of all the DTMF circuits I have designed, a big proportion was for security. Don't forget, I lived in South Africa for 21 years and Australians have no idea what crime is. I was the ' owner of a two-way radio factory, complete with security gates. Still I was robbed in my own office by five men with AK47s. That I am still alive is a miracle. I was also co-owner of a crystal factory and my partner was carjacked and then killed.

Radio is very important over there. When we had the first multiracial general elections, my lovely wife Beverley sold more than a thousand radios in a couple of weeks. In an emergency you have to transmit an ID code and we made masses of DTMF circuits. Works very well and is affordable.

The circuit of FIG 2 is our building block. Every incoming tone has its corresponding output high during that tone. Let me refresh your memory tone 0 = binary 10, * = Bll, # = B12, A = B13, B = B14, C = B15 and D = BO. We can make a simple two digit decoder, but there is a limitation, the two digits must be different, so 11, 22, 33 etc are no-nos, but that leaves us still with plenty of usable two digits to play with (FIG 3)

This is already a rather safe circuit, 8 and 2 switch a 555 on and 5 and 3 will switch it off. The values of C2, R8, Cl and R7 depend on the encoder used. If this is done manually and especially with a shaky hand, we have to make the values rather high.

The working is quickly explained
Output 8 will charge C2, input 2 of IC3 (a 4093 nand gate) is high, but starts discharge through R8. As long as pin 2 is more than about half of supply voltage, it is considered high. Output 2 (tone 2) will make pin 1 of 1C 3 high. Both gates are high and the output (pin 3) goes low and that is exactly what a 555 needs to switch on.DTFM Decoders and Encoders

1C 2 (555) is purely used as a switch, not as a timer. Pin 2 needs a negative pulse and pin 4 (reset) must be high. Pin 3 is the positive output, about 4.5 volts.

To switch off, we have to either make pin 4 (reset) low or 6-7 high. Output 5 will charge Cl and output 3 will make the other gate high, pin 4 of IC3 goes low and so will pin 4 of the 555, which switches off.

If the tones are made with an automatic encoder you can make the values of C2 and R8 (Cl - R7) reasonably low. The Cs are 1uF till 4.7uF and the Rs 1-3.3M, C 4 is 1OnF.

We don't have to use nand gates, it can also be done by inverters (40106). Let's go for three digits on and three digits off. On is 5-6-7 and off is 5-6-8 (FIG 4). In this circuit, tones 2 and 3 could be the same, as I built in a few tricks of my trade. It works as follows: Output 5 (of course from the 4515) will charge C5 (2.2uF). The discharge resistor (R19) is 1M - it gives us some time. Output 6 goes high now and will only make pin 1 of IC4 high if C6 is still reasonably charged. The value of R9 (2M2) and R19 (1M) are calculated in such a way, that output 6 can never make pin 1 high by itself. This principle can be used to create as many digits as you would like. Pin 1 high means pin 2 low, cap C6 will be discharged, all of this happens during tone 6. Normally pin 3 of IC4 would be low as well, but we put the strobe on that gate. Only when tone 6 has gone, pin 3 goes low and pin 4 high. The purpose of injecting the strobe is to be able to use the same tone again. The voltage on IC4 will charge C7 and everything starts all over again. Output 7 will make pin 11 high (If C7 is still more than half supply voltage, of course). Pin 2 of IC5 (555) goes low and output 3 goes high. Output 8 could switch it off under the same condition of C7

There is also a way of eliminating timing circuits, but that will be for next month, we will also give you an automatic 4 digit encoder, for which I have made a PC board.

Till next month


To Circuits Galore Pt 1

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