I'm trying to make an ultrasonic beam breaker circuit - a transmitter sends a continuous ultrasonic signal to a receiver, both placed in a straight line. When the beam is broken, a peizo buzzer turns on. When the beam is restored, the peizo turns off. I was thinking of generating the ultrasonic signal using a 555 timer, and found this. However, I have no idea on how to implement the receiver circuit.

Meanwhile, I found an alternative way to make this project here, but I can't understand the explanation provided.

So, how can I go about implementing the ultrasonic receiver circuit? Is the receiver circuit given in the Electroschematics website the only way, or is there an better, simpler way to do this?

  • \$\begingroup\$ Which part do you not understand, the transmitter or receiver? \$\endgroup\$ – Tim Mottram Apr 20 '17 at 11:04
  • \$\begingroup\$ what don't you understand about the receiver circuit shown electroschematics.com/5144/ultrasonic-beam-barrier - it seems very straightforward - sensor - amplifier (T1,T2) - rectifier - switch \$\endgroup\$ – JIm Dearden Apr 20 '17 at 11:06
  • \$\begingroup\$ Show whatever receiver circuit you are asking about. Otherwise, this question is too broad as we're not going to design a ultrasound receiver for you. We couldn't anyway since you've given no specs. \$\endgroup\$ – Olin Lathrop Apr 20 '17 at 11:10
  • \$\begingroup\$ @JImDearden I'm having trouble understanding the "signal detector" stage in the receiver - the one using T2, C3, T3 and the need for capacitors C3 and C4. I don't have much knowledge about transistor circuits other than switches and amplifiers. \$\endgroup\$ – QuantumCookie Apr 20 '17 at 12:19

The receiver:


enter image description here

T1 is an amplifier stage.

The detector stage:

VR (1M0) is adjusted so that without an input signal (beam broken) the collector of T2 is near 0V. (T2 is turned ON)

With no input signal (broken beam) the base of T3 is OV so it is turned OFF. The LED has no voltage across it so it is also OFF. T4 gets a base current through R7 and is turned ON. With T4 turned ON the buzzer sounds.

When the beam isn't broken there is an AC signal (through C2) into the base of T2. This causes it to switch ON and OFF at the ultrasonic frequency. D1 allows the positive half cycle to charge up C3 and C4 (through R5). This produces enough (steady DC) current to turn on the LED and T3. (although though I'm not convinced that the LED is in the correct place as the base emitter junction would clamp the voltage to about 0.7V - way below the voltage need for the LED )

With T3 turned ON, T4 is turned OFF so the buzzer doesn't sound.

Proposed modification to existing circuit: - improved LED indicator

enter image description here

I'm not happy with the position of the indicator LED so by adding an extra NPN transistor (as a simple switch) when the voltage across C3 is high (beam not broken) T5 turns ON and the LED is lit. You can use any colour LED.

When the beam is broken C3 decays rapidly, T5 and T3 are switched OFF

The 100k resistor is there to discharge the capacitors and to make sure T3 is fully OFF (pull down). I don't see C4 as being necessary.

  • \$\begingroup\$ So, the function of C3 and C4 is to give a steady DC voltage by reducing the "ripples" in the rectified AC voltage (You were right about the LED not getting turning on) ? \$\endgroup\$ – QuantumCookie Apr 20 '17 at 13:57
  • \$\begingroup\$ @QuantumCookie yes - because the frequency is high (40kHz) you don't need a large value capacitor (C3). Its basically the same as an envelope detector circuit used in am radio or a half wave rectifier with smoothing capacitor. As C4 is connected across the b-e junction its not really going to keep the transistor ON for very long despite its value. \$\endgroup\$ – JIm Dearden Apr 20 '17 at 17:41

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