Electronics > Beginners
Make a strobe which automatically varies between 8-20 Hz with a 555 chip?
seanspotatobusiness:
I tried to link to it via my OneDrive but now I've attached it to this post as well/instead. Changing C3 to 100 nF reduced the frequency to something I was expecting although it still switches instantly between frequencies in this simulation.
Edit: when I used C2 for the sawtooth, I had to use a diode to stop it from getting stuck at a full charge. Although the sawtooth is happening, the frequency from U2 appears to be fixed (right windows):
soldar:
It's late for me and I need to go but I made some changes which should get you closer. The signal going from the first to the second timer is a sawtooth now. See if you can advance some more.
Buriedcode:
--- Quote from: seanspotatobusiness on February 06, 2019, 10:50:22 pm ---LTSpice wouldn't launch on my own PC but it's running okay on my mum's laptop. Here's a link to the circuit on my OneDrive: https://1drv.ms/u/s!Avvwzet1PgNTgqV9UEgr8HoTQ46H6g
The frequency is too high at the moment. Also it seems that the circuit switches between two frequencies without a smooth transition between either. I was hoping to get the frequencies between 8 and 20 Hz as well as 8 and 20 Hz themselves. I'll play around with the component specifications and post again if I manage to get anywhere near my goal.
--- End quote ---
The reason is, you're using the output of the first 555 to control the thresholds of the second 555, and the 555's output is essentially a square wave - either VCC, or 0V. So the second 555 is being controlled by the first, and is just switching between two frequencies, one where CV is high, and one where CV is low.
In order to generate a "smooth" transition you would need to create a sawtooth wave - that slowly ramps up and down between two voltages rather than very quickly switch between VCC and GND. This is why I mentioned using comparators or opamps, but you can generate a sawtooth wave with the 555 - check the voltage on the "trig" or"thres" pin - you'll see a sawtooth wave between 1/3VCC and 2/3VCC (in your case, powering it with 15V it'll be between 5V and 10V).
The trouble with connecting this to the CV pin is it isn't a low impedance source - that is to say, the OUT pin of a 555 can source and sink quite a bit of current - it just connects to VCC or GND via transistors, and can supply >100mA. But the voltage node on the trig/thres pins is the capacitor voltage charging and discharging through R1 and R2. So if that was to be used, it would need a buffer - an amplifier with a gain of 1, so the output is the same as the input, but.. it can provide some current, and prevents the input voltage being pulled off.
The problem is, putting a signal on the CV can only change the frequency so much. 8 to 20Hz is a 2.5x range. A bit of experimentation shows the following attach LTspice circuit:
With the values shown, CV = 2V gives ~ 20Hz, and a CV = 10V gives ~8.2Hz. So we need to generate a sawtooth wave that varies between those two voltages. I don't think the 555 can't do this. So you can either try and generate the wave some other way - comparators, opamps etc.., or use something other than the 555 to generate the pulse train, some kind of voltage controlled oscillator, so that the required sawtooth wave voltage is different. There are a number of ways to do this.
Buriedcode:
Best I can do. First 555 creates a (nonlinear) ramp over about 9 seconds. If you wish to increase this you could increase R2 and R8 or C1. Because this voltage is needed to drive the CV pin of the next 555, it needs to be buffered, so a cheap LM358 as a voltage follower will do. As the voltage is between ~1.6 and ~10V, you could use almost any opamp as most have a common mode input within that range.
The other 555 just generates between ~9Hz and ~20Hz and sweeps between the two. That isn't 8Hz but its close. As you can buy two 555's in the same package - the NE556, its a two chip solution.
Notice the clever trick on the first 555 - the output is fed into the CV pin to change its internal thresholds, this gives a much wider range for upper and lower thresholds. Please don't assume this will work on a breadboard - the model of the NE555 in LTspice is "ideal", in reality the "trig" and "thres" pins are connected to comparators internally, and these may not be able to read voltages too close to VCC or GND. So if you can - prototype it!
seanspotatobusiness:
Thanks a lot. I look forward to doing this in a breadboard soon. I think I will need to buy a cheap oscilloscope since my local maker space is an hour away. I don't need to do any high frequency stuff so hopefully there are some threads on EEVblog discussing crappy oscilloscopes for hobbyists. Then again, maybe even a serial oscilloscope made using an Arduino would be fast enough for this.
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