Ultrasonic Receiver (range finder in listen only mode?)

Does anyone know of an Ultrasonic Range finder where you can put it into a listen mode without having to do a ping first as most of the ultrasonic range sensors I've tried only do a full ping/echo cycle. I'm looking to build a simple but cheap bat detector and ranger finders tend to be cheap and operate at the right frequency for a lot of bats, so if I could find an ultrasonic range finder that worked in an listen only mode that would be most helpful.

Thanks

What you're asking for is a microphone which will detect sound but there's no simple way of detecting distance. The ultrasonic range finder sends a ping and calculates the time for the ping to return. That's the only way it works.

All I want to know is if there is a bat in the area, ie I use the range finder in 'listen' mode so it picks up the 40kHz echo location ping from the bat. The problem I'm having with all the range finders I've seen so far is they only 'listen' after they send out a ping and then only for a very short time. So either I have to find a range finder that can be put into a permanent listening mode or can be modified to be in a permanent listening mode.

This might be useful...

http://home.earthlink.net/~bat-detector/ArduBat/

The most common bats in my area produce sounds at ~47 kHz, less commonly I see ~63 kHz and ~75 kHz and I've a few times picked up ~115 kHz sounds. This covers pretty much the complete range of bat frequencies. The first may record on the transducer of a typical 40 kHz ultrasound range finder such as the HC-SR04, the other frequencies almost certainly not. Many chirping insects also produce a good volume in ultrasound, typically around 28 kHz.

I don't know what my batscanner uses to detect the sounds but I think it are small electret type microphones.

Ohh there is an easy way … You use UART modes that is available in various ultrasonic sensors.

You can use this sensor

But I think you have to solder a jumper or a resistor on the back side of the board for it to work in UART mode

or you can use this one which already comes with a built in UART mode

In UART mode … you just have to connect POWER, GND and TX and you will get distance readings from the ultrasonic sensors every 100ms. No need to send any ping or anything from the micro controller. There is no need to do any calculations too. You just get distance in cm at fixed intervals.