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#95939
OK I have finally got my ultrasonic sensor circuit to work last night in its final state, which is to drive an outdoor transceiver, and detect the echo, then display it as an analogue signal through PWM and onto an LCD screen in distance. This is all working, however the distance it achieves is only 2 meters and I am just working on improving the effeciency and sensitivity to get 6 meters as you can see the 6 meter echo on the scope, but it just isn't big enough for a comparator to happily pick it up without noise interfering.

Anyhoo, besides that, I want to get my board size and cost down, and at the moment the single most expensive and big component (apart from the sensor itself) is the 1:10 transformer I am using to give me 50V drive (well above 80V thanks to the self resonance) from my 5V supply.

My question is, what type of components could I use instead of a transformer to give me a 1:10 increase of a 5V 40Khz drive signal that are SMD mounted ideally, that way I can also keep my board entirely surface mounted and single sided.
#95964
What drive signal do you need, and what is the load capacitance?

You can typically make a standard boost supply using an inductor, transistor, diode, and capacitor, and then switch this DC voltage to your load, but it may be adequate to produce high-voltage pulses directly; this can be more efficient than a DC supply (because the peak voltage can be significantly higher than the output from a boost circuit using the same inductor).

I have been playing with pulsed boost supplies based on components from dead compact fluorescent lamps. Using the output inductor from one of these (which isn't small -- about 10mH :!: ) I can easily get 200V peak voltages into a 2KΩ load using a 3V input.
#95980
The datasheet for the sensor is here: http://www.farnell.com/datasheets/524508.pdf

I currently run a 5V driving signal from a uC pin to the gate of a transistor that is on the 5V line and drives a 1:10 transformer at 5V 50mA on the primary, and gives a 50V drive on the secondary which has the transducer on it. Here is the current circuit (note there is actually a 1k resistor on the DRIVE1 pin coming from the uC to the transistor gate, I just forgot to put it in the schematic:

Image

I have not played with boosting voltage at all other than transformers so any links or guidance you can provide would be great. The idea about the 200V drive thing sounds good, what did you use to get that?

Luke
#96004
Just wondering about increasing RX sensitivity, rather than driving the transducer to the point of smoke...

Try a back to back diode between the xformer and the transducer. You will lose ~1V p-p drive, but the winding won't be loading the transducer during the receive phase...

-Mark
#96057
Here's a circuit that I tested in SPICE that puts out over 300V pulses from 5V (in simulation, anyway).

For each pulse into the gate of M1, L1 stores energy; when M1 is turned off the collapsing magnetic field causes a large positive voltage spike. D1 allows the first pulse through to your transducer, then isolates the transistor and inductor from your transducer so you can listen to it.

M1 is a high-voltage NMOS FET (high enough breakdown voltage for the voltages you want to put out).

Also, D1 should be capable of those voltages (the faster the better, but high-voltage beats speed; a 1N4007 would work OK here).

Ensure that the on-time of the pulses on M1's gate are short enough to avoid saturating L1 (shortening the pulses will reduce the voltage).

The components on the right side of the circuit are equivalent to your input circuit.

The inductor is a cheap radial 470uH choke about 9mm in diameter x 12mm tall. You could get smaller inductors if you didn't need that high a voltage.

I was playing with the much larger inductors out of compact fluorescent lamps.
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#96064
Mark,

Would putting the BAV in series with the receiver like that not just drop 1V off the entire line, meaning my amp would recieve 79V+ instead of its original intended job of limiting the amp input to +- 0.6V?

Also, the receiver is already at 7,500x amplification and I am having to take great care with my tracks to prevent intereference on the input line as even running a track under or within 2mm of the input/output lines causes the amp to rail. I have to keep the lines well apart and also apply laquer over the board to remove the noise so any more amplification and it is just not going to work. I am quite amazed that after driving the sensor at a good 90V and amplifying the echo by 7,500x that it is still hardly noticable after just a few meters! Yet the indoor sensors I have work fine at 5V and 7,500x amp, or 90V and 1,500x amp.


Mike,

Cool, never knew about voltage doublers before. Just reading up on them now sounds very good and easy to do with simple components. And with the current being low (presumably) at 50mA I shouldn't get too much voltage drop. I will read up more about them first so I understand it better but I presume pulsing the inputs to get a DC voltage and then pulsing that DC back to an AC through a transistor would be how it's done?


bikeNomad,

I think I understand your circuit, it is basically the same as my original one but replacing the transformer with a single inductor, and instead of the secondary pin of the transformer providing the voltage, that is instead coming off the bottom of the inductor through a diode to the transducer. And the reason for the diode is to prevent any echos coming back on the transducer from being sunk by the inductor/transistor? Hopefully I have understood that correct. Is there a specific calculation for the collapsing field of an inductor and its charge as to the voltage spike, so I can calculate a specific voltage?

Cheers for all the help guys.

Luke
#96078
One thing I didn't ask is how many pulses you're delivering per echo, and how far apart they are. You have to account for the discharging and recharging of the 100μF capacitor.
angelsix wrote:
I think I understand your circuit, it is basically the same as my original one but replacing the transformer with a single inductor, and instead of the secondary pin of the transformer providing the voltage, that is instead coming off the bottom of the inductor through a diode to the transducer.
Well, not exactly. Instead of a pulse transformer (where you can say you have a 10:1 transformer x 5V = 50V) you are actually using the inductor as a kind of spring, building up energy and releasing it suddenly. Inductors resist changes in current (like capacitors resist changes in voltage); when you interrupt the input current flow, the voltage rises to maintain the same current. Since you have a high impedance load, that voltage can rise pretty high.
And the reason for the diode is to prevent any echos coming back on the transducer from being sunk by the inductor/transistor?
Partly. And to ensure that you get just a single pulse. Of course, if you want more than one pulse you can add a diode in series between the transistor and the inductor (cathode to transistor), thereby letting the inductor "ring". Replace the output diode between the transistor and the transducer with a capacitor which will resonate with the inductor at 40Khz.
Hopefully I have understood that correct. Is there a specific calculation for the collapsing field of an inductor and its charge as to the voltage spike, so I can calculate a specific voltage?
It may be easier to breadboard this. Get some inductors and high-voltage FETs and sit down with an oscilloscope and a pulse generator and see what you get.

The problem here is that what limits your voltage tends to be things that aren't necessarily specified in the component data sheet.

For one thing, your transducer is more complicated than a simple capacitor; it takes energy to wiggle it mechanically (there are some interesting academic papers on modeling piezo transducers, by the way). Some transducers go seriously non-linear at a high enough voltage, suddenly conducting current instead of acting like a capacitor.

Then there's the "parasitic" properties of the inductor: parallel capacitance (which you can typically get close to by using the "self resonant frequency" of the inductor if it's given; the higher the SRF, the lower the capacitance).

And there's also the question of how much current you can flow through the inductor before it goes into "saturation"; this depends mostly on the specific core material and is often not well specified, though if you stay well below the maximum current given by the manufacturer you probably won't go into saturation. And if you power the inductor's field from a capacitor as in the diagram I posted, the parasitic "equivalent series resistance" and "equivalent series inductance" parameters are interesting, as they determine how fast the inductor can charge. As can the inductor's series resistance.

I've attached my SPICE model; you can run it using Linear Technology's excellent LTSpice http://www.linear.com/designtools/software/ltspice.jsp simulator.
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#96082
Thanks I have just downloaded spice and will take a look, and I will have to do some good reading up and learning on the whole inductor/diode thing to fully understand it as it is a little beyond my knowledge at the moment, and I will also try mocking it up on a breadboard at the same time just to see how it reacts and I may learn that way too.

I have been meaning to get into SPICE but have not had the time so I will have a go of that over the weekend too. Thanks.

Luke
#96188
Hi angelsix,

Your current design always has the (transmit) transformer in parallel with the transducer, so when the transmit pulse has finished, the receive path may not be sensitive as it could be.

The addition of a back-to-back diode between the transmit transformer and the transducer is designed to isolate the two when in receive mode so this loading does not occur.

I am not suggesting that you remove the existing back-to-back diode across the receive input.

Hope that makes sense.

Mark

============

Would putting the BAV in series with the receiver like that not just drop 1V off the entire line, meaning my amp would recieve 79V+ instead of its original intended job of limiting the amp input to +- 0.6V?

Also, the receiver is already at 7,500x amplification and I am having to take great care with my tracks to prevent interference on the input line as even running a track under or within 2mm of the input/output lines causes the amp to rail. I have to keep the lines well apart and also apply lacquer over the board to remove the noise so any more amplification and it is just not going to work. I am quite amazed that after driving the sensor at a good 90V and amplifying the echo by 7,500x that it is still hardly noticeable after just a few meters! Yet the indoor sensors I have work fine at 5V and 7,500x amp, or 90V and 1,500x amp.
#96413
Mark,

Just to let you know I added the BAV to the transformer and I think it gave a slightly more sensitive reading than before, possibly a foot or 2 more on the distance which is good so I've added it to the future boards.

Now I have a working sensor for up to about 3 meters for the unknown one, and a good 6 meters for the prowave one, but still want to get it to all SMD stuff.

I think I will try the suggestion by bikeNomad next as I have all the components I need, and see what I can come up with that way, and will also investigate the voltage multiplier and let you know the results.

Luke
#96415
Luke,

Well done.

Have you tried tuning the TX transformer - you might be able to squeeze out more juice.

Try 47pF, 100pF, 220pF or 470pF caps across the secondary and see what happens. If each of them reduces sensitivity then you may have a good match already. If you see an improvement with any of them then that means you may be able to improve things further by bringing the transformer to resonance. Try different vales, or combinations.

Regards,

Mark