Description.
Here is the circuit diagram of a very simple and accurate electronic compass using the 1490 digital compass from Dinsmore. The 1490 digital compass is a solid state Hall Effect device which is sensitive to detect the Earth’s magnetic field. The sensor is internally damped to simulate the performance of an ordinary compass. The circuit shown here is the simplest one that can be built around this sensor.

The sensor has 12 pins at the bottom which are arranged in to four sets of three. All pins marked 1 are connected to the positive (+5V). All pins marked 2 are tied together and connected to the ground. The pins marked 3 are the output pins. Each output pin is equivalent to the open collected of an NPN transistor and can sink up to 20mA. An LED is connected between the positive (+5V) and each of the output pin. The 1K resistor will limit the LED current to 5mA.The direction can be identified by interpreting the combination of LEDs glowing. A typical 5V regulator using IC 7805 is used to produce 5V DC from the 9V battery for powering the circuit.

For the calibration of the compass circuit, you need an ordinary magnetic compass. Determine the North using the standard compass. Now, rotate the compass so that only one LED glows. Take this LED as the North. The sensor does not have a fixed North and you can actually use any output as the North. If the circuit is wired exactly as show in circuit diagram, the other three outputs will also fall in the proper sequence. In my circuit the LED D4 was the North. Rotating the digital compass in the clock wise direction will make the LEDs to glow in the following sequence where 1=ON and 0=OFF.

Circuit diagram.

Notes.

• Assemble the circuit a good quality PCB.
• Using sub miniature LEDs will increase the battery life.
• The circuit can be powered from a 9V PP3 battery.
• Ferrous materials close to the sensor will affect its performance.

Description.
The circuit is nothing but two LEDs (D1 and D2), whose status are controlled by the temperature of the surroundings. The famous IC LM35 is used as the temperature sensor here. Output of LM35 increases by 10mV per degree rise in temperature. Output of LM35 is connected to the non inverting input of the opamp CA3130.The inverting input of the same opamp can be given with the required reference voltage using POT R2. If the reference voltage is 0.8V, then the voltage at the non inverting input (output of LM35) becomes 0.8V when the temperature is 80 degree Celsius. At this point the output of IC3 goes to positive saturation. This makes the transistor Q1 On and LED D1 glows. Since the base of Q2 is connected to the collector of Q1, Q2 will be switched OFF and LED D2 remains OFF. When the temperature is below 80 degree Celsius the reverse happens.IC1 produces a stable 5V DC working voltage from the available9V DC supply. If you already have a 5V DC supply then you can use it directly.

Circuit diagram.

Notes.

• The circuit can be assembled on a Vero board.
• IC3 must be mounted on a holder.
• The temperature trip point can be set by adjusting POTR2.
• Type no of Q1 and Q2 are not very critical. Any general purpose NPN transistors will do it.

Description.

This simple circuit will produce an alarm whenever the temperature falls below zero degree. A thermistor is used here to sense temperature. The op-amp LM7215 is used to compare the reference voltage and voltage from the thermistor network. Reference voltage is given to the non inverting input (pin3) of the IC and voltage from thermistor network is given to the inverting input (pin4).When temperature becomes less than zero degree the voltage at the non inverting input becomes larger than the voltage at the inverting input and the output of the op-amp becomes high. This makes the transistor Q1 ON and drives the piezo buzzer to make the alarm. In the power supply section, IC 7805 is used to derive 5V from the 9V battery.

Circuit diagram with Parts list.

Power supply for this circuit.

Notes.

• Assemble the circuit on a good quality PCB.
• The thermistor used here is a glass bead thermistor, type No: KEYSTONE RL0503-5536K-122-MS (361K @ 0 degree Celsius and 100K @ 25 degree Celsius).
• The IC1 must be mounted on a holder.
• The battery B1 can be a 9V PP3 battery.

Description.
This is a simple and low cost wide band VHF field strength meter. The field strength is measured by converting the radio signal to DC and measuring it. The RF signal will be picked up by the coil and rectified by the diode D1.Even a very small DC voltage is sufficient to alter the biasing of FET and it will be reflected in the meter as an indication of the presence of a radio signal. The meter can be calibrated by adjusting the preset R2 to make meter M1 read ZERO in the absence of any radio signal. This circuit is not very sensitive, but can sense radio signals from hand held FM transmitters up to a distance of few meters( ideal for theoretical demonstrations).

Circuit diagram with Parts list.

Notes.

• The circuit can be assembled on a general purpose PCB.
• Use a 9V PP3 battery for powering the circuit.
• Use a 250uA FSD current meter for M1.Using a lower FSD meter will improve sensitivity.
• The coil L1 can be made by making 6 turns of 20 SWG enameled copper wire on a ¼ inch plastic former.
• The antenna can be a telescopic whip antenna.

Notes.

This is a very simple circuit that can be used to sense electromagnetic radiations. The circuit can even detect hidden wrings. A 1mH inductor is used for sensing the electric field. The electric field will induce a small voltage in the sensor inductor and this induced voltage is amplified by the opamp.The headphone connect at the output of the opamp will give an audio indication of the electric field. For example, the electric field around a mains transformer can be heard as a 50 Hz hum. The POT R4 can be used to adjust the gain of the amplifier. By keeping the sensor inductor near to a telephone line, you can even hear the telephone conversations.

Circuit diagram with Parts list.

Notes.

• Assemble the circuit on a general purpose PCB.
• The circuit can be powered from a 9V PP3 battery.
• It is better to have a radial type inductor for L1.
• The POT R4 can be used to adjust the gain.
• The switch S1 can be a slide type ON/OFF switch.
• The IC1 must be mounted on a holder.
• All electrolytic capacitors must be rated at least 15V.