By Phil Ray electron@ieee.org 1-Nov-1999

INTRODUCTION HARDWARE SOFTWARE OPERATION IMPROVEMENTS

HARDWARE

The following paragraphs describe the various circuits used by the project. With the exception of the FM transmitter circuit, (which was purchased in kit form and built with the included components) component values were chosen mostly based on what I had on hand. Other values can be substituted- within reason. Note that the interface is simple enough to be constructed with point-to-point wiring in the prototype area on the DT-104 SimmStick board!

Refer to figure 1, the schematic of the thermometer interface.

The audio interface is designed to couple a portion of the 0-5v square wave generated on PD6 into the audio input of the FM transmitter. C101's purpose is to block DC from flowing through the voltage divider composed of R101 and R102. R102 provides a way of setting the modulation level to a comfortable level and allows plenty of adjustment in case another transmitter design is substituted.

The power interface, which allows the '1200 to switch the FM transmitter on and off, consists of a single 2N3906 transistor (Q101) and a 110k ohm resistor (R103) to limit base drive current. A relatively high value of base resistance was chosen to reduce power consumption when the transistor is on. Note that a logic LOW on PD5 turns on the PNP transistor and allows current to flow to the transmitter. The software design takes this into account and sets PD5 HIGH soon after a reset on the micro-controller is received. The FM transmitter circuit used in this project draws only a few milliamps when powered up, so a power transistor is not required.

Interfacing the DS1620 is a simple matter of connecting the DQ (bidirectional serial data), CLK (clock), and /RST (reset) pins to PD2, PD3, and PD4, respectively, on the '1200. The chip draws its power from the system's regulated 5v supply.

Figure 2 shows the modifications made to the Velleman K1771 "FM Oscillator" kit to allow it to be used with this project. The PC board contains a microphone amplifier which was not required. Other kits or prebuilt transmitter units could be easily substituted.

The SimmStick DT-104 board from Dontronics holds an ATMEL AT90S1200, a 500KHz ceramic resonator with associated capacitors, a low-power 3-terminal voltage regulator, reset circuitry, and an RS-232 interface (not used here). My interface is built onto the blank prototyping area on the right hand side of the board.

A close-up of the prototyping area shows the Dallas Semiconductor DS1620 Thermostat/thermometer chip at top left. The audio interface, with its 10K "Modulation Level" adjustment, is at top right. The transistor and resistor make up the power interface at the bottom.