Introduction
The goal of this project was to build a small, cheap, light-weight transmitter to attach to a small animal. This version uses a commercial, low-power transmitter, the Radiotronix RCT-433-AS. The design worked well, but has a relatively high current draw of about 500 microamps. A CR1620 lithium coin cell runs the circuit for about a week. The current version uses a pair of CMOS oscillators to produce a chirp once per second.
The Circuit
The circuit shown below has two standard CMOS multivibrators. The second is gated by the first. The duty cycle of the first oscillator is about 1%, or 10 mSec every second. A logic-high at the transmitter control turns it on.
Physical layout of the circuit
The circuit board was layed out using ExpressPCB software. You will need to download a copy to view the design file. The components are all surface mount. The dots shown below are on a 0.1 inch grid.
More photos
Note the antenna lead at the lower left. At 433 MHz (70 cm wavelength) a quaterwave antenna should be 17 cm long. The battery is partly shown at the right. It is a CR1620 lithium cell, but any 3 volt source may be used. The transmitter module sticks out to the left.
