Project E - P2

It has been a few weeks since my last update about Project E. Project E is a personal project to develop a range finder. It is a self improvement thing for me.

I initially had a problem with my amp. It is a 2 stage inverting amp. I could not get it to amplify the rx'd ultrasonic signal. The other problem was there is no way to know if my tx is working .

The only way I could find out if I had any signal at the rx was to check it out using an oscilloscope. I tapped the signal before it entered the amp and the pic below was what I obtained.


1. No obstacles in front of the tx and rx. Small distorted signal in milivolt range.

2. Next, I tried with a book above the tx and rx. Finally a semblance of a response. Notice the difference? The signal got really thick and messy. That was how I know that my Tx is working.

So something is wrong with my amp. Which I later found out was due to my negative voltage supply. It was supplying a mere -3V instead of -5V. The next step was to find a fix.

I was initially using a MAX232 with its charge pump function to generate -5V. However it turns out that the MAX232 is unable to drive any load higher than 20kohm. My friend at work pointed it out to me (felt like a duche-bag for not reading the datasheet properly).

I ditched the MAX232 and tried a dual output voltage network (recommended by my friend). It didn't work...well, it did get me +5 and -5V. However it had to be connected direct to my source and thus my source will be rendered +5V. Connecting +5V to other devices will create uneven drain on this network and will then affect the -5V output. So this method was ditched too.

In the end, I managed to source a negative voltage converter (TL7660CP) from Texas Instruments. It just needed 2 electrolytic caps to make it work! Simplicity in itself!

I ordered some samples from TI and it arrived in less than 2 weeks! See below for some pictures.

3. Can't wait to open it. This is what came out of the envelope.

4. ESD safe packaging (nothing less expected).

5. Finally got to trying it out on my circuit. There are 2 LED's in the pic. The one on the right (lit up) shows that the Tx is working. The one on the left is the LED connected to the amp output of the Rx. So if everything works, that lower left LED should light up and blink. ;)

6. IT WORKS!!! You can see the left LED lighting up after I put a book above the Tx/Rx! Finally a step forward for my project! I've learned lots! It is never too late to learn!

Power consumption at this point is 51mA (according to the DC power supply).

Next step would be to figure out the programming to calculate the range and output through PIC the distance from obstacle.

:)