jjimenez101

Hello everyone,

I am using a 555 timer to generate a 48HZ square wave with 65% duty cycle. Thats about a 20.8 ms period ( 1/f) with 13ms high and 7.8ms low. That square wave coming out of the 555 timer will be fed into 2 other 555 timers. One timer will generate a pulse width of 1ms, and the other a pulse width of 2ms. This is to control a servo motor by the way.

My questions is:

1) Being that the 555 timer can act only as a non-retriggerable one shot, will it be a problem for me? Or should I have to use a retriggerable one shot like the 74HC123 for this application?

By the way I cannot use a micro-controller for this application, that is why I am using 555 timers instead of generating the desired signals through a controller.

Thanks-
-George-

 

it should work just fine. mono mode.

how are you switching between the two output 555s? reason I ask is if you are using a switch, why not just use a 555 in async mode and a spdt to switch between 2 resistors that give you 1ms or 2ms high pulse. Then you could do the whole thing with a single 555. The actual period is not critical for hobby servos. if you use a diode between threshold and discharge, you can keep the period fairly constant. You might have to take care to avoid glitching the output frequency.

 

I would use an oscillator, counter and gates.

I don't know whether you want exactly 7.8 ms low, but if it can be 8 ms, then all you need is a modulo 21 counter. Make the oscillator 1 kHz and thus the period (at the output of the counter) will be 21 ms. Then use gates to decode the counter outputs to give you the 1 and 2 ms outputs that you need.

If you do it this way, then the only adjustment necessary is to set the osc frequency (or use a crystal and a suitable counter to divide it down)

If you really need the 7.8 ms low to be exact, then you will need a modulo 104 counter driven by a 5 kHz oscillator (0.2 ms period).

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Len

 

jjimenez101

Let me rephrase what I said in my original post. I actually will be using a micro-controller. But not to generate the signal. The controller will be used to select which 555 timer output I want, either the 1ms or 2ms PW. The output of the 2 555's will be fed into a multiplexer, the select pin of the multiplexer will be connected to the controller, this controller will select which PW will be at the output of the multiplexer by either sending a 1 or a 0 to the select pin. So the controller will do the switching.

The 7.8 ms low is not what I am aiming for here. The end result that I want is one signal from one 555 timer to have a period of 20ms, with high time of 1ms. The other 555 timer should have the same period, but 2ms high time. Then like I said they will be fed into a multiplexer. Then I will switch between these signals to move the servo motor wither full left or full right.

With that said, does it clarify to you guys what I'm trying to do? And is there any problems with using this approach?

 

Check out this thread for a circuit schematic I posted that I use to drive servos ...

http://www.electro-tech-online.com/e...96-servos.html
JB

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Nothing is impossible for the man who doesn't have to do it himself - Weiler's Law

 

Then all you need is an oscillator, modulo 20 counter and 2 AND gates. You don't need the MUX as the output selection can be done by one input of the AND gates. The advantage of this approach is that you only have to adjust the oscillator to the frequency. If you have 2 555s then both have to be adjusted. Also, the 555s need several discrete components around them.

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Len

 

Here is a suggestion

Attached Images

Pls gen.gif (50.3 KB, 33 views)

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Len