Ever wonder what animals hear that you don't? Normal hearing ends at about 15Khz to 20Khz there are a lot of sounds you cant hear like a dog whistle or a bats chirp.
Here is a little receiver for the ultrasonic range that will help you out. It's local oscillator was meant to run at 35Khz which will be subtracted from the incoming sound making a 40Khz tone into a 5Khz tone. Something you can easily hear. Technically the mixer also produces the sum but for the 40Khz example that's 75Khz and way outside what we care about. I probably should filter that out....
It has other uses like finding leaks in air or vacuum lines and finding arcing insulators on power poles. These things make ultrasonic noise too. Even rubbing two quarters together will make a lot of ultrasonic noise. I wish I could find one of those old TV remotes that had two buttons and were just tiny ultrasonic tuning forks inside.
Step 1: Schematic
The schematic is pieced together from many different designs I have seen over the years. I always wanted to build the one I saw in ARRL's QST magazine in April 2006 from W1TRC. It used a discrete Fet mixer stage. I recently was working on an HF radio project and got some extra NE602 chips. That revived my interest in this project but using this chip rather than a fet.
For bench use the lower section with the 555 timer can be removed and you can use a signal generator to feed the mixer. This gives you a wider range on tuning and a more stable oscillator.
The LM386 is an old standby I had plenty of from headphone amp projects.
I built it as three separate sections on the board to make it easy to stop and restart as time permitted, Luckily I've recently started to organize my spare parts so building things is getting easier. Hopefully that means more instructables.
Step 2: Building It
I used a small protoboard and a BNC connector I had salvaged from an old Network Card.
The caps came from an assortment I had in the parts room.
I used old pc connectors for the I/O leads. this makes replacing controls easy if needed.
I added a charging plug so i can use a 9V nicd battery later on.
I put all the jacks on the end so its easy to store away.
I will make a PCB template for it soon.
Step 3: Front Panel
I used the Gimp to make a front panel image. I printed it on an Avery clear label sheet. If it was for a plastic panel I'd have used a paper label but with an aluminum plate the clear has a better look. Just scuff up the plate with a Scotchbrite pad first. I usually coat the finished panel with a couple of coats of Krylon clear but I'm out of that right now....
Step 4: Input Choices
There are plenty of choices for input. I have 4 input pins shown in the main schematic. Shown here are ways to connect different sensors, How you wire it and which pins get used depends on the sensor you pick and whether or not you use shielded cable. I did find one 3 pin sensor. the 3rd pin was for feedback when used with an oscillator but it seemed to work well with a differential input.
In the end I used some 40Khz transducers I got from old 1970's ultrasonic alarms. They are nice because they have a small phono jack on the back making a nice remote probe.
I also tried some old earpieces from telephone handsets. If you want to use an Electret Condenser microphone I added power on pin A.
In the finished prototype I connected C&D together and tied them to the barrel of the BNC connector. I then tied Pin B to the center pin of the BNC connector. I put a small 2 pin jumper on the board to tie Pins A&B together to add phantom power if need by the mike.
I've got to dig out a Piezo Tweeter and try that on the input.
Step 5: Finished Product
The first real use for this is to find a leak in my truck... Then I'll listen for Bats in the yard. I've been told they're out there. Truthfully I don't want to know...
You could even hook up a condenser mike to the input and a Piezo Tweeter to the output and talk to the Bats. I wonder if they understand DSB?