Oscillator - Double Square Wave Pulse Signal with Pause

[JSmith1957]

Hi All,

New to the forum. I have only some hobby experience with oscillating circuits but this one is over my head.

I need an oscillator that will trigger two opto-couplers (H11D1). I need them to be triggered in the following sequence:

"opto 1 - pause - opto 2 - pause - pause"

The above sequence should be a 500 Hz cycle (1.67ms), with each pulse and pause of equal duration (approximately 0.33ms each in this case). The main point is to have opto 1 and opto 2 pulse for the same amount of time (with a slight pause between them) and a long pause between opto 2 pulses and opto 1 pulses (as noted above).

The attached image shows two options I've been working on. The first wave form would be supplied by a single oscillator while the second wave from would be supplied by two in phase 555 timers, though I have no idea how (or if its possible) to get them in phase.

Please let me know if anything is unclear as I may have not explained my goal very well, but I'll try to fill in the blanks.

One side note, I'd like to be able to adjust the frequency between 200Hz and 1000Hz, but this is not necessary. The main problem is with the oscillating circuitry.

Thanks for any help in advance.

JSmith

[pebe]

You could use a 555 to drive a 4017 decade counter. Each pulse would step the counter through its 10 steps.

If you connect output Q5 to the reset pin, it will then reset every time the count gets to 5, so will count in sequence 0,1,2,3,4,0,1,2,3.4,0 ..etc.

Set the 555 up to give a pulse to the 4017 every 33ms. You can then take your first output from Q0 and the second from Q2, to give:- pulse1 - nc - pulse2 - nc - nc - pulse1 - etc.

[JSmith1957]

You could use a 555 to drive a 4017 decade counter. Each pulse would step the counter through its 10 steps.

If you connect output Q5 to the reset pin, it will then reset every time the count gets to 5, so will count in sequence 0,1,2,3,4,0,1,2,3.4,0 ..etc.

Set the 555 up to give a pulse to the 4017 every 33ms. You can then take your first output from Q0 and the second from Q2, to give:- pulse1 - nc - pulse2 - nc - nc - pulse1 - etc.

Many thanks Pebe. I'll put the setup together as you have directed.
One quick question though; I took a look at the 4017 datasheet. Does this device give nice square waveforms? It looks like the timing diagrams show a gradient transition between pulses. Would a PWM chip (like SG3524) be better. Again, I don't know how to hook this one up either actually....

Thanks again,

[pebe]

I'll never know why datasheets show waveforms with sloping sides, rather than sharply rising, as they usually are. If you look at the datasheet you will find the transition times vary with supply voltage, and at 10V they are 100ns for the rising edge and 50ns for the falling edge. Plenty fast enough for your 300µs pulses.

The SG3524 won't be any good for you because you cannot get the pauses between pulse that you want.

The attached circuit should be OK for you. Timing is by C1 and R1, and with those values will give a clocking period of 0.31ms. If you want to change it you can calculate it with:
period=1.4xC1xR1

You haven't said what you want to drive from the pulses, so remember that power from the 4017 is limited and you may need transistor buffers.

[JSmith1957]

Pebe,

Understood, I'll continue with the 4017. Your schematic will save me a lot of time, though looking at it I think I can easily minipulate it from here as it makes pretty clear sense.

Definitely understand about the need to amplify the output current.

I'll work thorugh the circuit and let you know the result.

Thanks agian for your help.