Nuts and Volts - July/August 2018

the appropriate modes, range, and pulse characteristics. For the transmitter (as with the 555 implementation), the output period can be controlled by a photoresistor as one leg of a voltage divider.

However, with this circuit, you can just as easily drive the off-time input from a voltage source. For instance, you could use a photocell whose output voltage varies with light intensity.

Schematic 12A is an example of the transmitter using a PIC 555 replacement and a photocell. The circuit is essentially a VCO with a 50%

duty cycle. The placement of the photocell may look a little strange, but keep in mind that lower voltages on the control inputs correspond to lower period values, i.e., higher frequencies.

Putting the positive terminal of the cell on the +5V line will cause the control voltage to decrease with higher light intensity which will then yield higher frequencies.

Schematic 12B shows a similar circuit using the NCO feature of a PIC18313. (See Part 3 of this series for sample programs.) In this case, however, higher voltages on the control input cause higher frequencies on the output.

In both cases, you’ll want to make sure that the maximum output voltage of the photocell is less than 5V. Resistors R5 and R8 are there to limit the current to the inputs in case of excessive photocell voltage.

For those of you who like a programming challenge, you can edit MSMVx2.asm and modify it so that one section is an astable multivibrator and use it to drive the IR transmitter. The other section would stay as a monostable to be used as the receiver.

Mims Circuit 28

The last circuit in the book (Schematic 13) is a DC-DC converter. It uses a 555 in its astable configuration at about 2. 9 kHz with a low pulse width of about 7 µs. This is very close to a 98% duty cycle.

In my experimenting with the circuit, I found that I could not even get close to Mr. Mims results using the values and configuration in his schematic. I had to make some changes to get nearly the same results.