A Way-Better Variable-Frequency PWM Using Two TLC555 Timers (or One TLC556)

I have seen many implementations of variable-frequency PWMs using a single 555 timer IC with two diodes to time both the frequency and the pulse width. They sort-of work. In this Instructable, I show how to make a variable-frequency PWM with two TLC555CP timers, one to control the frequency timing and one to control the pulse width. Adjusting one timer does not affect the other timer, which is what we want.

The first timer runs in astable mode, producing the frequency. The second timer takes its trigger signal from first timer, and it is running in monostable mode. It provides a one-shot timed pulse for each trigger pulse it receives.

1. Two TLC555CP Timers - These are Texas Instruments timers. They are better than the NE555P timers. They are built internally with MOSFETs instead of BJTs. They switch at frequencies up to 2.1 MHz and use less power. The pin-outs are the same. The NE555P timers will work for this project, but I have wound up the TLC555CP timers to over 1.1 MHz.
2. Four 1K resistors, ½ W
3. Four Potentiometers - Two 100K and two 10K
4. A set of non-polarized capacitors - This implementation and demonstration uses a 47 nF, a 10 nF, and a 100 nF capacitors, standard 50V. The pulse width timer will usually use a capacitor ¼ to 1/5 the capacitance of the one used for the frequency timer.
5. Oscilloscope
6. Breadboard
7. Wire for breadboard
8. 12V Power Supply

Download the pdf attached to the Introduction part of this Instructable. It is the circuit diagram. Notice that the two timers are wired together almost identically. The difference is that pin #2 of the frequency timer is connected to pin #6 and is therefore self-triggered, astable mode. In the pulse width timer, pin #2 is connected to the output pin #3 of the frequency timer, monostable mode. The trigger coming from the first timer is essentially an inverted trigger in that the voltage is normally high, and when the voltage drops low, it triggers the pulse width timer on the leading edge of the voltage drop.

Notice also that I have attached pin #5 on each timer to a 10K potentiometer, the CONTROL pin. This allows adjustment of the voltage at pin #5. This gives you a broad range of adjustment of both the frequency and pulse width. I use this potentiometer for gross adjustment of the circuit, and I use the 100K potentiometer for fine tuning of the circuit. In most implementations of timing circuits using 555 timers, pin #5 is attached to a capacitor and attached to ground. This is a waste of the pin #5 functionality.

When you need higher frequencies, swap out the two timing capacitors for smaller capacitances, and conversely, put larger capacitors for lower frequencies.

Let me know if you build the circuit, if you like it or don't, and if you find any errors so I can correct them.