Handy Electronic Circuits

PICmicro

Projects

Web Design

Re-Radiating GPS Antenna

The signal strength of a portable navigation system such as TomTom, a Garmin eTrex or a PDA/mobile phone with built in GPS receiver is often poor inside a car because of the metal coating which is often applied to the windscreen of a modern car. A possible solution is an active external antenna. However many portable GPS receivers do not have a connection for an external antenna. A re-radiating aerial improves GPS reception without having to physically connect an antenna to the receiver. A loop antenna is placed behind or in front of the navigation system. A loop is antenna is used for the radiating element. This is made from a sturdy piece of enamelled copper wire soldered to the PB board. The wire is 19cm long, equal to the GPS signal’s wavelength. It is normally recommended to keep the loop between 1/8 and 1/4 of the wave length to prevent self resonance. L1 is an air-cored coil, made by winding a few turns (6 or so) around a small diameter drill bit (diameter 5mm). Keep the indivdual turns apart by at least the thickness of the wire used. The active GPS antenna used for this project is an ordinary commercial type. The 5v can be used as the power supply for the GPS receiver or some other device with GPS functionality.

Mike Preamp and Squarer

The circuit triggers automatically in response to a sound or noise. The circuit comprises of an audio preamp, squarer and a sensitivity control. Instead of using an Electret microphone, triggering could be done with a PIR(passive infrared) motion sensor. The first half of the LM358 comparator is the sensor signal preamp and the other half of the LM358 comparator is the sensor signal squarer. T

Cooling Controller

The LM335Z ia device specifically designed for temperature sensing, its voltage varies linearly and quite accurately with its temperature. The LM335Z voltage interfaces to a LM393 comparator's inverting input, this voltage is compared to the reference voltage at its non-inverting input. A potentiometer is used to vary the reference voltage acting as temperature control to determine when the comparator output changes state. The output of the comparator could be used to control any temperature related project. For instance the comparator output could drive a transistor switch which in turn control a relay.

Select by trial and error VR1. A value of 500 Ohms is suitable for low value temperatures.

Expanding the PIC’s Output Lines

In cases when a microcontroller has insufficient output lines using a 74LS138 can be the prefered option instead of upgrading the microcontroller and incurring a greater cost. The 74LS138 is a 3-to-8-line decoder, with seven outputs that are high and one output that is low. The output that is low is governed by the binary code on the 3-bit input. This arrangement allows 3 microcontrollers output lines controlling 8 output lines.