Hey electrical /diy/er's. I don't post here often, but I've been doing some tinkering, and right now I have a need for a digital oscillator (5V, square/ pulse output), and I want it to sweep its frequency range upon pressing a button.
In other words, what I want to accomplish is this: when I press a button or something, the oscillator will start at a relatively high frequency (sub 60 Hz), and then it will ramp down and after a few seconds it will be at a low frequency (maybe around 0.5 Hz or less, or completely stopped).
Because this VCO will be routed to some standard 7400 series TTL IC's, I would ideally like to modulate only the output frequency, and keep the 5V amplitude constant.
An astable multivibrator seems like a good starting point (maybe I'm wrong, I am only a neophyte). Pic related is a good ol' 555 timer. Currently, the output frequency is easily adjustable with the 20k Ohm pot, and I'm happy with the range of frequencies at the high and low ends.
I'm wondering if maybe I can make some sort of voltage-controlled resistor rig out of a transistor, and then using a cap to control the ramping-down speed. I could then control the output frequency of the oscillator. But I don't know how to go about this.
Very sorry about the long post, but if you can help me accomplish my goal from the second paragraph, then please guide me, sensei.
>>909224
Basically the opposite of a chirp circuit then? It does the opposite of what you're describing.
555 has built in VCO circuits that most people don't notice. Just build the standard astable circuit and connect your control voltage to pin 5.
4046 is another simple option. It's a common chip with a VCO on it. The circuit is as simple s they come.
>>909226
Not totally sure what you mean by a chirp circuit, I tried to google it but I was just getting 555 "cricket" type buzzers. Which operate at a constant frequency
>>909230
Neat, I'l check out that other IC when I can get my hands on one. In the mean time, I'l try experimenting more with pin 5. In the OP pic, its just coupled to ground with a .1uF cap.
I'm just not sure how to get the sort of control voltage signal I want. I'l be playing around with it though, thanks
>>909236
> I'm just not sure how to get the sort of control voltage signal I want.
What your asking for is called a ramp circuit. I've drawn a picture for you. Pressing the button discharges the capacitor to ground. When the button is released the cap will start charging slowly through the resistor, producing a rising voltage.
In the circuit I attached the output voltage will rise following the y=ln(x) curve. This is probably fine for your project If you want the output to increase linearly you need to replace the resistor with a current regulator. You can accomplish this with a PNP transistor like 2N3906 and three resistors.
>>909246
Alright, thank you very much. I figured I would need something along those lines. That characteristic curve should work.
I'll just have to keep playing with the circuit. I haven't had much success implementing that circuit with the output going to pin 5 of the 555. But I'l keep on trying, its definitely a good starting point.
>>909302
How did you know that was the project I'm working on?! Great detective skills dude. And thank you very much, that is awesome.
>>909385
figured anything that requires a spindown circuit is usually a roulette wheel or a dice type circuit.
>>909446
Fair enough, thank you again. I couldn't find any 4017 IC's, and parts are a little hard to come by where I am.
I do, however, have a 7490 decade counter, the only caveat is that it outputs BCD numbers 0-9. Then I think I can use pic related to decode the BCD values to individual outputs.
>>909611
A roulette wheel that outputs its number in binary would be cool though.
>>909655
I might try it, in conjunction with a 7447 decoder to drive a 7 segment display... but then its not really a roulette wheel by definition :P
>>909660
something like this
https://www.youtube.com/watch?v=W6cOu8MA6zM
led dice
https://www.youtube.com/watch?v=ljwBh8YSK_k
>>909709
thats so awesome, I wonder what those chips were
Sorry to revive this, but I finished the circuit. I ended up using a 7447 and a seven segment display for the output. It works great, and I even have the DC buzzer making little "tick" sounds with each count.
The odd thing is, it always ends on a "1" with the spin counter engaged.
If I remove the capacitor (C3 in >>909302 ) it just acts as a super fast clock pulse and the circuit essentially becomes a random number generator.
put a huge cap across the 1 and 8 pins of the 555. the rail voltage jumps around depending on what digit is showing on the led. you could try isolating the power to the led display from the rest of the circuit.
