Ultrasonic Tx Rx Robotic Car

Hi All

I have a 555 sending a 40Khz signal (square wave) to my Ultrasonic Tx and I am receiviing and amplifying the signal with an LM358 ... How do I get a high/low when the signal is present and the the opposite when the signal from the Tx is absent ... this cct is going to be used for a robotic car ... the PIC is going to sense if an obsticle is in the way or not.

Thanks

Mr_Sparkle said:

Hi All

I have a 555 sending a 40Khz signal (square wave) to my Ultrasonic Tx and I am receiviing and amplifying the signal with an LM358 ... How do I get a high/low when the signal is present and the the opposite when the signal from the Tx is absent ... this cct is going to be used for a robotic car ... the PIC is going to sense if an obsticle is in the way or not.

Click to expand...

You can rectify and smooth the amplified signal, this gives a DC voltage that represents the strength of the received signal. You can then use a comparator to give a logic HIGH/LOW output, with the other comparator input fed from a preset, so you can adjust the trigger point.

Hi Nigel

I tried rectifying the signal but the problem is that the smoothing capacitor stays charged even after there is no signal at the ultrasonic Rx ... I am using a 0.47uF capapcitor ... the signal I am getting from the Ultrasonic Rx is a 200mV 40kHz sine wave ...


does anyone know of a site with a working Ultrasonic Tx Rx

Thanks

Mr_Sparkle said:

Hi Nigel

I tried rectifying the signal but the problem is that the smoothing capacitor stays charged even after there is no signal at the ultrasonic Rx ... I am using a 0.47uF capapcitor ... the signal I am getting from the Ultrasonic Rx is a 200mV 40kHz sine wave ...

Click to expand...

It's simply a standard AM detector, you need to design the time constant of the smoothing based on the carrier frequency used (so it removes it), and on the modulation frequency - which must be passed.

There needs to be a discharge path for the capacitor (and 0.47uF sounds excessively large anyway!), it's usually a resistor directly across the capacitor. Adjust these values until you get the speed of response you want.

An old LM358 is a poor choice for an ultrasonic receiver because its output can't slew quick enough and it has poor high frequency gain.
Its max output swing drops off above only about 5kHz, and it max gain at 40kHz is only about 10.
Many more modern opamps have full output swing to 100kHz and higher, and have much more gain at 40kHz.

Besides having more gain for a greater range, a modern opamp can also be used as an active rectifier circuit to cancel the forward voltage of a rectifier diode for greater sensitivity. :lol:

Mr_Sparkle said:

does anyone know of a site with a working Ultrasonic Tx Rx

Thanks

Click to expand...

edpcompany.com has automotive sonar packages. I bought one of thier Mini A or S series (not sure which one I have yet, bought it yesterday) at a surplus shop for $8.95. It seems to do detection on a "hit vs. miss" basis.

And parallax has a free Tx Rx schematic somewhere in their Basic Stamp pages. This one was an actual rangefinder and it has a resolution of about 2 inches as-is. This one requires a 40KHz crystal and a quad op-amp.