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  1. #1
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    Long distance ultrasonic range finder

    I am trying to adapt an app note AN2548 (Ultrasonic Range Finder with a tiny817), for this project and need someone I can ask a few questions.

    1) The product will use 3 weather proof ultrasonic sensors that have been picked by the customer.

    2) The product needs to detect a range of at least 18 ft and it would be good if I could go down to 2 ft without any gain changes.

    3) The product needs will sample at a 15Hz rate of measurement. The sensors will not overlap so they can be driven simultaneously.



    I have attached 1 page of the schematic implemented on the first turn of boards. The initial detection was not sensitive to get even a few feet with the speced sensors

    So I have tried a 25V supply and now have a excitation voltage of around 55V. This does concern me a bit from a safety perspective, but this is commercial not a customer product.



    Questions:

    1) Am I on the right track? Is there a better circuit?

    2) As I push the excitation voltage higher I am not sure how to determine if I am violating the specs of the transistor in the front of the input chain.

    3) With the current changes, detailed in the schematic, I am up to 10ft but have a ways to go, so need ideas how to get another 8ft

    Thanks in advance for any help

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  2. #2
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    Re: Long distance ultrasonic range finder

    Your schematic should work fine at 18ft. I've seen even simpler schematics that works at that distance.
    If the transducer and the RX active filter are on 40kHz, the TX driver filter (C16, L2) resonates on wrong frequency (61kHz).
    So, you have to change the C16 value to 20nF, to make the circuit resonate on 40kHz. The transducer has 2.5nF capacitance, which is in parallel to C16.
    Also have to verify (using a signal generator) that the fc of the built active filter is also 40kHz. If is a bit off-frequency you loose sensitivity (less distance).
    The maximum drive voltage of transducer MCUSD16A40S12RO is 60V.



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    Re: Long distance ultrasonic range finder

    Did the OP mention MCUSD16A40S12RO? It's an non-hermetical sensor, probably more sensitive than enclosed types.



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    Re: Long distance ultrasonic range finder

    Thanks for the response. I am using a different sensor out of China that I think is from the car industry. It comes with the JSN-SR04T-2.0 detector board, it is not as sensitive. The SR04T is driving this sensor with 70V. Can I measure the sensor capacitance accurately with a capacitor meter? I did check the frequency the SR04T is using, and it is 40Khz. I originally had a 10nF cap and 680uH inductor but was not able to drive it as hard before the signal would start to fold back. Could this have been the inductor only having a 40ma rating?



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    Re: Long distance ultrasonic range finder

    In addition to needing more gain/sensitivity I need to know if raising the excitation power source to 25V and getting a 55Vpp drive is causing voltages to the input transistor that violate it's spec.

    Thanks



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    Re: Long distance ultrasonic range finder

    I need to know if raising the excitation power source to 25V and getting a 55Vpp drive is causing voltages to the input transistor that violate it's spec.
    Transistor Q4 is driven into BE junction breakdown, even if is not damaged you'll reduce the sensor voltage this way. I wonder how you arrived at the present receiver schematic. Most circuits that I know have a diode clamp circuit with series resistor in the receiver channel.

    Can I measure the sensor capacitance accurately with a capacitor meter? I did check the frequency the SR04T is using, and it is 40Khz. I originally had a 10nF cap and 680uH inductor but was not able to drive it as hard before the signal would start to fold back. Could this have been the inductor only having a 40ma rating?
    Sensor capacitance measured at low frequency doesn't count for the LC circuit resonance. The sensor has real impedance around the resonance frequency. You should tune the drive frequency and the LC circuit for maximal ultrasonic output.

    Inductor rating has two aspects, continuous current rating and core saturation current. The continuous current rating can be exceeded for pulsed operation.



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    Re: Long distance ultrasonic range finder

    The schematic is a direct copy from an app note. In the app note the drive voltage was only 5V, but I have a sensor that needs a much higher drive voltage. I do not understand the location of the diode clamp. With the drive voltage at R17 changed to 25V the drain on Q3 will swing to 55V when Q3 is off and be at ground when Q3 is on. The base at Q4 will be very close to the same level. So are you proposing a diode in series with a resistor between Q4 base and C24, or in series with R24 or R29?

    Thanks



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    Re: Long distance ultrasonic range finder

    I suggest to study high voltage ultrasonic echo sounder circuits to get an idea how to implement voltage clamping in the receiver path. They often use a pair of antiparallel diodes limiting the amplifier input to +/- 0.6 V. A series resistor drops the high voltage.



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