Designing an Ultrasonic Range Finder with a PIC Microcontroller

[counterstruck]

Hi

I am designing a ultrasonic range finder. I am part of design group with the challenge of building a robot that can find its way through a known course.

I found some circuits avaliable on the net. There is a section on the transmitter circuitry that confuses me. I am using microcontroller to generate a 0 - 5V 40KHz burst of pulses. I want to boost the signal to have a larger voltage swing, and current drive to achieve larger power transmission hence further distance. I can amplify the voltage to 0 - 9V, but now how do I achieve a -9V to 9V swing?

The circuit uses hex inverters. Doesn't a hex inverter represent a logic 0 as ground? The explanation given about why two hex inverters are in parallel is still not clear to me.

I am using a PIC microcontroller, which drives out about 20mA. I want to achieve the max power transmission for a 40KHz piezo transducer. For current amplification, how does one design a current amplify for a -9V to 9V signal?

 

[berkeman]

Thread moved to EE forum. Welcome to the PF, counterstruck.

The paralleling of the inverters is meant to boost the current driven into the piezo element. With two inverters in parallel, you get about twice the drive current. There will be slight mismatches in timing for the two parallel gates, but if they are in the same IC package, the mismatch shouldn't make much difference.

Since each side of the piezo is being driven with a 9Vpp signal, and since the two signals are out of phase, that results in a 2x9Vpp = 18Vpp differential signal being driven across the piezo element.

BTW, can you tell us a little about safety considerations for medium-power ultrasound transmissions near humans? What power levels and duty cycles are considered "safe"? That should be a part of your research and report and construction -- making sure that you are building something that is safe to operate around unprotected humans. (Or at least understanding what precautions and protections you should use if you need to use higher power levels in your ultrasound ranging devices.)