Power input is to a 7805 5 volt regulator. A pair of LEDs is connected between the 5 volt supply and ground, with current limiting resistors in series and one pin on the AT90S2313 shunts the current through one LED or the other. When one LED is on, the device is "ready", that is not generating pulses on the output and not measuring input frequency. When the other LED is on, the devise is actively generating pulses or measuring frequency. As a consequence, one of the LEDs is on at a time allowing quick visual confirmation that power is applied and the state of the device. The serial interface is the AT90S2313's internal UART connected to the outside world via RS-232 using a Maxim MAX232 interface chip, which derives RS-232 drive voltages from the 5 volt supply.

The diagram below shows 5 switches connected to the 5 input lines of the parallel port. An external 5 volt power supply is used to provide high logic levels to the input pins when the switches are open. Three 1.5 volt batteries in series can be used to obtain 4.5 volts which is close enough. The 330 ohm resistors in series with the port connections provide some protection in case a connection is made to the wrong pin. If you are sure of the connections, the 330 ohm resistors can be left out and the switches connected directly to the input pins. The negative side of the power supply should be connected to the ground point, or any pin from 18 to 25.

In intrinsically safe applications, it is sometimes necessary to separate sections of circuitry by resistors which limit current under fault conditions. The circuit shown provides an accurate analogue output with effectively zero output impedance, despite having resistors in series with the output. which is independent of RA and RB.

A 3-x -3-inch piece of circuit board, or similar size metal object which functions as the pick-up sensor, is connected to the gate of Ql. A 100-MD resistor, R2, isolates Ql"s gate from Rl, allowing the input iropedance to remain very high. If a 100MO resistor cannot be located, just tie five 22-MO resistors in series and use that combination for R2. In fact, R2 can be made even higher in value for added sensitivity.

The power supply has been simplified. Power transformers and rectifiers have been omitted and some parts have been omitted from the MOSFET voltage regulator circuits: 1N5242 zener diodes between the source and gate and 10k resistors in series with the gate. These parts serve as protection in case of accidental short circuits, but don`t affect the

The power supply has been simplified- Power transformers and rectifiers have been omitted and some parts have been omitted from the MOSFET voltage regulator circuits: 1N5242 zener diodes between the source and gate and 10k resistors in series with the gate. These parts serve as protection in case of accidental short circuits, but don`t affect the operating point.