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# can anyone help me understand the design of this circuit

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#### alfie.max15

##### Member level 2

Can anyone please look into this circuit and tell me its working...
i am weak in circuit design and i am trying to make a ultrasonic distance measurement device.

It's really two circuits divided by the ground connecton in the middle.

The top is a signal generator, you have to produce pulses in software to drive it. The bottom is a receiver, producing a signal the software can read.
Your software would normally produce a burst of pulses at ultrasonic rate (typically 40KHz) and then start a timer. The timer stops when the receiver section picks up the echo. From the time delay you then work out how far the sound travelled and divide it by 2 to get the distance to the reflector.

You should note that with the values shown, the transistor is always conducting. It would be better to design it so the transistor only conducts when necessary.

Brian.

It's really two circuits divided by the ground connecton in the middle.

The top is a signal generator, you have to produce pulses in software to drive it. The bottom is a receiver, producing a signal the software can read.
Your software would normally produce a burst of pulses at ultrasonic rate (typically 40KHz) and then start a timer. The timer stops when the receiver section picks up the echo. From the time delay you then work out how far the sound travelled and divide it by 2 to get the distance to the reflector.

You should note that with the values shown, the transistor is always conducting. It would be better to design it so the transistor only conducts when necessary.

Brian.

Thanks for the reply.... but i know this much of about the circuit...
but what i want to know is the design of the circuit...
i mean the significance of the values of each components and the usage of the LC circuit..
and other parts of the circuit...

actually it's a very strange design.

The typical 40KHz piezo Tx unit will have a capacitance of ~ 1nF - 2nF. With a 330uH inductance this does not resonate at 40KHz. Better would be to connect the piezo between collector and ground to take advantage of the inductance flyback for higher voltages. Once again, the max voltages of the common piezo units is around 25-30v, so care is required not to exceed this too often.

In addition, while an LC sounds interesting to use here to get higher voltages, but for distance measure we actually want the TX to stop oscillating quicklyonce the drive signal is removed. Otherwise objects close by will get hidden by the TX still vibrating.

The diode also seems to serve no purpose. It certainly does not protect the bjt from high flyback voltages.

The choice of a BD135 which is medium power is unusual. Here a simple 2N3904 or similar/ smaller bjt can be used.

Lastly - as betwixt also pointed out - your uC will never be able to switch the transistor OFF. R2 should connect to GND, or not be used at all.

Perhaps the original design didn't use one of the conventional 40KHz piezo devices. The guess at the frequency was mine, not the original posters although I've never seen one that works on anything but 40KHz. Possibly it used a small open disc as used in lower frequency alarm sounders.

I *think* the intention of the diode is to stop the transistor damping the oscillations when it turns off but it never would anyway! My guess is this design is someones idea that never reached the debugging stage.

Brian.

edit:

Just looked at the original article and it does use Murata 40KHz transducers so as mentioned, it isn't going to be very efficient either in performance or electrical efficiency.

i'm sure it has been tried out.

Trust us, its a bad design. Other than it's only good feature : that it is very very very simple - connects directly (almost) to the uC.

There are plenty of much better circuits & designs out there. Look for them.

alfie.max15

### alfie.max15

Points: 2
i'm sure it has been tried out.

Trust us, its a bad design. Other than it's only good feature : that it is very very very simple - connects directly (almost) to the uC.

There are plenty of much better circuits & designs out there. Look for them.

i have looked at many but only problem is that i just cant make anything seeing the circuits...
can anyone suggest some...
as i have said i am trying to make an ultrasonic distance finder

i just cant make anything seeing the circuits...

which part are you having difficulty with ? which circuits have you looked at ? Some of them are just a little more complicated (i.e. a few more components) but will work well.

this is one of them... as you said the designs are complex and i basicly am weak in circuit designing and understanding its working...

That's a MUCH better design although it lacks resistors in all but one of the LED segment connections. Its range might be improved slightly by adding a resistor or better still an inductor across the piezo loudspeaker but even as it is, I would think it's range would be many times that of the original design. To give you an idea, I worked on something similar a while ago which used only one transducer to transmit and receive and it had a measurement range of about 60 metres.

Brian.

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