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Old 04-19-2010, 01:17 PM
espanol090 espanol090 is offline
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Default square wave voltage level shifter

Hello ,

i m trying to make a circuit that basicly generates a square wave and this wave should pass through LED- AİR- FOTODİODE and we have to see that signal from out of the circuit. but i have some questions . if you could help me , i d be very thankful. i attach the circuit . i m using multisim 10.1 .

1- my objective to make this circuit is i want to transmit a any signal with using led and fotodiode. i mean there ll be a short air space between them and led flashes off and fotodiode recieves it and transmit at the other side of the circuit. is it possible ? ( no optocoupler ) i have to do it for real )
2- as you can see in my circuit , lm555 generetas a square wave ( the wave above) . but the problem is i can t correct the voltage level of the signal . i mean i can calibrate frecuency but i can not play with the voltage level except Vpp . but i m not interested in changing Vpp . i want to put it between -5v and 5 volt. how can i do that ?
3- and secondly, the below signal, it s way down from the original signal that enters the opamp. how can i make it rise to the orignal or any level signal that i want ?

these are the questions that i cant find answers. thanks.
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Old 04-19-2010, 02:26 PM
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SgtWookie SgtWookie is offline
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Your R2 is much too low in value. You would burn up a real 555 timer. A basic "rule of thumb" I use is a minimum of 100 Ohms per 1v of Vcc, so you would need at least 1.2k Ohms for R2.

To keep the 555 output more symmetrical, you can then increase R3 to 91k, and decrease C2 to 0.2uF.

R4 is 52 Ohms; that is too low. If the Vf of the IR emitter in the optocoupler is 1.3v (fairly typical for an IR LED) and you want about 10mA current flow through it, then:
Rlimit >= (Vsupply - Vf_LED)/10mA
Rlimit >= (12v-1.3v)/0.01 A
Rlimit >= 10.7/0.01
Rlimit >= 1070 Ohms. 1.1k Ohms is the closest standard value >= 1070 Ohms.

R5 is far too low of a value. It's best to limit the maximum output current of an optocoupler to around 5mA, unless they are specifically designed for more current.
R5 >= 12v/5mA = 12/.005 = 2.4k Ohms.

C1 is 40 FARADS! You would need a big truck to carry it around in.
Reduce it to around 0.1uF

That will get you started at least.
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