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I have constructed a Pulse width modulating circuit that should output an amplitude of 10 Volts PWM signal. The control input signal is a 5Vpp, 10KHz sine wave and a pulse input of 10Vpp, 100Hz square wave.PWM circuit I know that the VCC value controls the amplitude of the PWM output signal. However, I tried setting it to 10 volts and the oscilloscope did not display any signal. The only time the oscilloscope displays an output is when the VCC is set to 6.5 volts and below.

PWM output using 6.5V VCC. The red signal is the PWM signal, the blue square wave is the pulse input and the green signal is the control signal.

Any thoughts as to why my circuit does not output an amplitude of 10V PWM signal? Please help me solve this problem.

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  • \$\begingroup\$ Examine your design specs and list them. fo, deviation , linearity then examine internal design of 555 , Vdc.bias for each port and how to achieve linear deviation using a sawtooth that matches the upper and lower thresholds. and the RC values needed for the signals to be <<5% of input impedance. Here is a crude example with an internal oscillating mode that you may shift up by reducing the external RC osc cap. tinyurl.com/y5eh28o2 \$\endgroup\$ Nov 4, 2020 at 4:29

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I don't know if this is the only problem with your circuit, but the trigger threshold (assuming a bipolar 555 like LM555) is 1/3 of VCC (Actually lower and variable due to your connections to CON, thanks to @Jasen for pointing this out).

If you increase VCC without changing the amplitude of the trigger input, eventually the trigger input will never go low enough (because you are ac-coupling and re-biasing to Vcc) to be registered as "low" and the 555 will never see low-to-high transitions of the trigger.

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  • \$\begingroup\$ In jamie's circuit the threshold is variable centered on 1/4 VCC \$\endgroup\$
    – Jasen
    Nov 4, 2020 at 8:17
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With the circuit you give increasing VCC to 10.7V should give 10V output amplitude

If you want to use the same 5V input signal you'll need to change most of the resistors.

to get the same timing at this higher voltage increase R2 (try 8.6K)

to get the same threshold on the control voltage pin reduce R3 (try 4.7K or 3.3K)

To get the trigger level in the same ballpark as with the 5V circuit add a resistor from trigger to ground. (try 2.2K, possibly increase R4 to 1.5K or 2.2K)

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