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You can pick up a 3½-digit digital volt-meter module nowadays for a little as a couple quid. This is a simple and expensive way to fit out a piece of equipment with an instrument. Most modules are based on the well-known ICL7106 IC . They operate from an ordinary 9-V battery, and they only provide a fixed measuring range (200 mV or 2 V). The accessory circuit described here converts a DVM module into a voltmeter with 20-V and 200-V measuring ranges, with the added bonus of automatic range switching. This requires a ground-referenced symmetrical supply voltage (±5 V) instead of a battery. An inexpensive TL431C is also used to generate an adjustable reference volt-age from the supply voltage. The circuit described here uses an LCD module with a fixed measuring range of 200 mV. It has three pins for driving the decimal point; two of them are used here. Automatic Range Switching Schematic This is how the circuit works: IC1 converts the voltage to be measured by the DVM m...

The post explains a 10 km long range transmitter circuit using ordinary components .PCB fabrication also explained. With the increased activity on 2 metres, some who may have contemplated working this band, have possibly been deterred by the cost of a " black box " and the uncertainty of modifying commercial equipment. With this situation in mind, the Author has , designed a simple , easily-built 2m f.m. transmitter, that can be assembled by anyone who can use a soldering iron and small tools whilst possessing a reasonable amount of patience. The completed unit will perform well, being as versatile as the constructor ultimately wishes to make it. An r.f. output of around 1 watts can be expected if the unit is constructed as described but power far in excess of this may be, achieved if the design is regarded as a working basis and the p.a. stage is developed. Construction Procedures for the proposed long range transmitter circuit A simple printed circuit technique is employed...

The use of transmitters which have a more powerful output than the ‘flea-power’ are sometimes required when there are many obstacles in the path of the surveillance transmitter and monitoring station receiver, or the distance between them is too far so as to make a low powered device feasible. Whereas a typical micro transmitter will produce an RF power in the order of just a few milliwatts, i.e. a few thousandths of a watt, the VHF-FM transmitter described has a power output of between around a half and 2 watts, depending on the power source, which may be anywhere between 6 volts and 30 volts d.c. . Circuit diagram : Long Range FM Transmitter Circuit diagram The battery or batteries should be of the alkaline high power type, since the current drain will be found to be relatively higher when compared to microtransmitter current drain The power output of this device is somewhat proportional to the current drain and so therefore both may be decreased by altering the value of R6 to a...