I have this dual 556 timer that i have wanted to use, just because it can take 15V and work with out an issue and as low as 3V. (thanks TI) So i was thinking that this had some potential to be used for RF or a fixed PWM signal. (personally i would like to pick up a few digital POTs and a digital cap to to create a DCO of some sort)
So far i have played with it and found some interesting things out.
My first goal was to get a 50% duty cycle from the chip, this was pretty easy just choosing the correct values for Ra and Rb, but you will have to do some math and you should use the equation out of the data sheet for your chip. My equati0n Period = tH + tL ~= CT (RA + 2RB) In 2, but if you need values closer to what is measured, you will have to break out the TI 8x and plug in tH = Ct (Ra + Rb) ln [ 3 – exp(-t PHL / Ct (Rb + Ron) ) ] + t PHL and Tl = Ct (Rb + Ron) ln [ 3 – exp(- t PHL/ CT(Ra + Rb)) ] + tPLH. I know these equations look difficult but as long as you plug in your values you will get close to what you measure on your oscilloscope. but i did this the fun way, use the first equation to get close to what you need then tweak the rest of the values until you get something close to what you want.
The values that i chose first were the Ra and Rb, I chose Ra = 220 ohms and Rb = 100 ohms and Ct = .0047 uF. Rl and Cl i did not choose a value nor did i hook up those components, i will have to look into the reason they are there, but i’m sure they are there for good reason. This first Test was just to make sure i can get a good square wave and be able to shape it.
My second goal was to try and make a sine wave or shape the square wave to be better for RF transmission. I added a Pi Network to the out put to shape the wave and below is what i got for half of the pi network, i will have better pics of the final wave form once my circuit is a little more complete.
If all goes well and i can create a pretty stable transmitter i will be hooking up a small modem chip to the circuit and create a cheap digital modem . Another thing that would be nice would be a pair or transmitters and receivers so i can send and receive at the same time.
R1 100 ohms
R2 220 ohms