I am trying to send a burst of PWM pulses between a separate ultrasonic transmitter and receiver (stripped down a cheap HC-SR04) .
Sending a perfect squarewave with 50% duty will be received and amplified perfectly up to 16 meters using a multistage setup. However, when I try to send a burst with 25% or 75% duty, it will still output aprox 50% duty on the receiving end with/without transistor/op amp setup using my oscilloscope. I have verified with oscilloscope that the arduino output pin have the correct duty cycle.
Now I am beginning to think that the transmitter or receiver automatically transforms the incoming signal to an outgoing 50% duty signal?
Any suggestion what might be wrong?