Secret Diagram

This circuit measures the distance covered during a walk. Hardware is located in a small box slipped in pants' pocket and the display is conceived in the following manner: the leftmost display D2 (the most significant digit) shows 0 to 9 Km. and its dot is always on to separate Km. from hm. The rightmost display D1 (the least significant digit) shows hundreds meters and its dot illuminates after every 50 meters of walking.

A beeper (excludable), signals each count unit, occurring every two steps. A normal step was calculated to span around 78 centimeters, thus the LED signaling 50 meters illuminates after 64 steps (or 32 operations of the mercury switch), the display indicates 100 meters after 128 steps and so on.

For low battery consumption the display illuminates only on request, pushing on P2. Accidental reset of the counters is avoided because to reset the circuit both pushbuttons must be operated together. Obviously, this is not a precision meter, but its approximation degree was found good for this kind of device. In any case, the most critical thing to do is the correct placement of the mercury switch inside of the box and the setting of its sloping degree.

The recent tsunami in Japan and the on-going calamity with the Fukushima nuclear power plant has apparently greatly increased sales of radiation meters, not only in Japan but elsewhere around the world. This device will allow an estimation of the level of radioactivity, being sensitive enough for background radiation monitoring or to provide an estimation of the level of radioactivity from sample objects such as Thorium gas mantles in LPG lamps. The circuit is compatible with several Geiger Muller tubes and three types of indication are provided: the good old-fashioned audible click with each discharge, a flashing LED or an analog meter providing a rough average of radiation levels.




A normal background count in New Zealand with the smaller GM LND712 tube is around 30 counts per minute, while the larger and more-sensitive LND7312 pancake tube will count about four times this figure. Both GM tubes will detect alpha, beta and gamma radiation. Unless the tube is “filtered”, there is no way of knowing just what type of radiation is being detected, although a rough guess can be made. Alpha particles will be stopped by placing a sheet of paper between the tube and the source, Beta particles (electrons) will be stopped with a few layers of aluminium foil and the more lively Gamma rays will need a layer of lead.

The circuit provides a regulated 500V supply for the Geiger Muller tube. This voltage places the tube into its linear operating mode so that a discharge inside the tube will occur when a particle enters through the mica window of the tube and causes the gas to ionise. The very short pulse produced is stretched and used to signal that a discharge has occurred. The power supply consists of an oscillator and small transistor driving the 6V secondary of a 240VAC mains transformer. The stepped up output of the transformer is fed to a Cockroft-Walton voltage multiplier consisting of diodes D3-D7 and the associated 47nF 630V metallised polyester capacitors.

IC1 is a 40106 Schmitt trigger inverter and IC1a is connected as an oscillator running at several hundred hertz. This is buffered by IC1b and fed to the base of NPN transistor Q1 which then drives the abovementioned transformer. IC1c acts as an error amplifier to regulate the high voltage fed to the GM tube. A portion of the DC voltage produced at the junction of diodes D4 & D5 is monitored by a voltage divider consisting of the 4.7MO and 47kO resistors, in combination with trimpot VR1. When the voltage from D5 is below the positive threshold of IC1c, its output will be high and IC1a will be able to oscillate. Hence, the oscillator will pulse on and off, to maintain the 500V set by VR1.

Each time there is a discharge in the GM tube, the resultant current triggers the BT149 SCR which discharges the associated 100nF capacitor and thereby acts as a pulse stretcher to drive the three remaining inverters in IC1. These in turn drive a high-brightness red LED (LED1), a piezo transducer and an analog metering circuit which is based on an old VU meter movement with a scale graduated in counts/minute. The current drain of the circuit is 10mA and a small 9V battery should run the counter for many hours. Warning: do not touch the window of the GM tube. These are very fragile and made of very thin mica, to allow the low-energy alpha particles to pass through. With the LND 712, 200 counts per minute is roughly equivalent to 0.3 micro-seiverts.

Here is the circuit diagram of a seven segment counter based on the counter IC CD 4033.This circuit can be used in conjunction with various circuits where a counter to display the progress  adds some more attraction.

IC NE 555 is wired as an astable multivibrator for triggering the CD 4033.For each pulse the out put of CD 4033 advances by one count.The output of CD 4033 is displayed by the seven segment LED display LT543.Switch S1 is used to initiate the counting.Diode D1 prevents the risk of accidental polarity reversal.