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# Connecting input to TTL

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##### Full Member level 3
Hi,

Just a simple query. If I have a input signal of voltage from 5.8-5.2V, I would want to connect it to a TTL chip and have the above input signal act as 0 for 5.2V and 1 representing 5.8V. How should I do it?

THanks for your help!

A simple level translator can be build with a 1kΩ resistor and a 4.7V zener diode connected in series with the Base of NPN transistor ..
- any voltage below 4.7V+0.65(Ube) ≈5.35V will "close" the NPN
- any voltage above 5.35V will "open" the NPN
So, if you connect Emitter to 0V and a 10kΩ resistor between Collector and 5V, the collector voltage will be TTL H for input<5.3 and TTL L for input>5.3 .. (if you need you can use an inverter, or another NPN, to change the otput to L for input<5.3V and H for input>5.3V) ..

Reagrds,
IanP

Points: 2
Hi

First of all, it would be good to analyze the signal you have, I mean does it only take values of 5.8V and 5.2V? doesnt it take values such as 5.4, 5.5 or 5.6V? If the answer for my second question is yes, then you need to define a threshold of your signal for which it can be represented by "0" or "1" by the TTL circuit. The reason for this, is shown in http://www.interfacebus.com/voltage_threshold.html, where for TTL an input signal is safely assumed as "1" when it is greater than 2V and it is assumed as "0" when it is lower than 0.8V, it means that if TTL can not assure you what will it be assumed, if any, in the case 1.5V is inputed to the circuit.

A safe way to do the interfacing is employing the comparator LM339 http://www.onsemi.com/pub/Collateral/LM339-D.PDF

see you

Sal

Points: 2
THanks

For slow signal you can use optocoupler .

Hi,
just put a diode clamp to VCC it will work

Just a simple query. If I have a input signal of voltage from 5.8-5.2V, I would want to connect it to a TTL chip and have the above input signal act as 0 for 5.2V and 1 representing 5.8V. How should I do it?

THanks for your help!

Dear friend
Use a comparator with open collector on the output like lm339 ( don t use opamps ) and put a reference voltage + 5.5 v on inverting terminal and put your signal on non inverting terminal.make the supply of the open collector terminal = 5 v and makevcc for the comparator any value between 6 and 24 v it will work fine
regards

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