Simple BFO Metal Locator

This circuit uses a single coil and nine components to make a particularly sensitive low-cost metal locator. It works on the principle of a beat frequency oscillator (BFO). The circuit incorporates two oscillators, both operating at about 40kHz. The first, IC1a, is a standard CMOS oscillator with its frequency adjustable via VR1. The frequency of the second, IC1b, is highly dependent on the inductance of coil L1, so that its frequency shifts in the presence of metal. L1 is 70 turns of 0.315mm enamelled copper wire wound on a 120mm diameter former. The Faraday shield is made of aluminium foil, which is wound around all but about 10mm of the coil and connected to pin 4 of IC1b.

Circuit diagram:
Simple BFO metal locator circuit schematic

The two oscillator signals are mixed through IC1c, to create a beat note. IC1d and IC1c drive the piezo sounder in push-pull fashion, thereby boosting the output. Unlike many other metal locators of its kind, this locator is particularly easy to tune. Around the midpoint setting of VR1, there will be a loud beat frequency with a null point in the middle. The locator needs to be tuned to a low frequency beat note to one or the other side of this null point. Depending on which side is chosen, it will be sensitive to either ferrous or non-ferrous metals. Besides detecting objects under the ground, the circuit could serve well as a pipe locator.
Author: Thomas Scarborough - Copyright: Silicon Chip Electronics
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Neon Flasher Runs From 3V Supply

A neon indicator typically requires at least 70V to fire it and normally would not be contemplated in a battery circuit. However, this little switchmode circuit from the Linear Technology website ( steps up the 3V battery supply to around 95V or so, to drive a neon with ease. The circuit has two parts: IC1 operating as step-up converter at around 75kHz and a diode pump, consisting of three 1N4148 diodes and associated .022µF capacitors. The 3.3MO resistor and the 0.68µF capacitor set the flashing rate to about once every two seconds. The average DC level from the diode pump is set to about 95V by the 100MO feedback resistor to pin 8. The circuit could also use an LT1111 (RS Components Cat 217-0448) which would run at about 20kHz so L1 could be reduced to 100mH and use a powdered iron toroid core from Neosid or Jaycar.

Circuit diagram:
neon flasher circuit schematic
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Low-Cost Dual Power Supply

This circuit shows how to symmetrically split a supply voltage using a minimum of parts - one LM380 power amplifier plus two 10µF capacitors. It was originally published in National Semiconductor's AN69 and provides more output power than a conventional general-purpose op amp split power supply. Unlike the normal power zener diode technique, the LM380 circuit does not require a high standby current to maintain regulation. In addition, with a 20V input voltage (ie, for ± 10V outputs), the circuit exhibits a change in output voltage of only about 2% per 100mA of unbalanced load change. Any balanced load change will reflect only the regulation of the source voltage, Vin.

Circuit diagram:
Low-cost dual power supply circuit schematic

The theoretical plus and minus output tracking ability is 100% since the device will provide an output voltage at one-half of the instantaneous supply voltage in the absence of a capacitor on the bypass terminal. The actual error in tracking will be directly proportional to the unbalance in the quiescent output voltage. An optional 1MO potentiometer may be installed with its wiper connected to pin 1 of the LM380 IC to null any output offset. The unbalanced current output is limited by the power dissipation of the package.

In the case of sustained unbalanced excess loads, the device will go into thermal limiting as the internal temperature sensing circuit begins to function. And for instantaneous high current loads or short circuits, the device limits the output current to approximately 1.3A until thermal shutdown takes over or the fault is removed. For maximum output power (2.5W), all ground pins (3-5 & 10-12) should be soldered to a large copper area (the LM380 data sheet contains more details).
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Cheap AC Current Measurement

The easy way to measure high AC currents is to use a clamp meter but these are generally quite expensive and cost several hundred dollars at a minimum. Add-on clampmeter adaptors can work well but they only work with digital multimeters which have millivolt AC resolution. This is because the output of most clamp adaptors is quite low, 0.1A = 1mV, for example. This is no good for typical cheap DMMs which have a lowest AC voltage range of 200V. This circuit can be built into a low cost clamp meter such as the Digitech QM-1565 from Jaycar Electronics. When dismantling this clamp adaptor, remove the label which has the AC range conversion factors and then undo the two screws gain access to the inside.

Circuit diagram:
Cheap AC current measurement circuit schematic

The two cross-connected transistors act like low voltage drop diodes to generate a DC voltage which is proportional to the current in the primary of clamp adaptor (ie, the circuit under test). The recommended transistors are power germanium types such as ADZ16, AD162, AD149, ADY16, 2SD471, OC16 and OC28. This approach gives lowest voltage drop and good linearity, from 10 to 300A. Schottky power diodes can also be used but the result will not be as linear. To calibrate, wind 10 turns through the clamp adaptor's jaws and feed a current of 20A through the winding. This is equivalent to a single turn carrying 200A. Set the trimpot to suit your multimeter, normally set to the 2V DC range. Do not calibrate for a low current otherwise accuracy at high currents will be poor.
Author: Gerard La Rooy - Copyright: Silicon Chip Electronics
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Simple Cat.5 Network Tester

This circuit came from a need for a "quick and dirty" network tester that could be operated by one person. All the commercial units I tried required a person at the other end to check the remote LEDs, as the transmitters could not be made to cycle through the test continuously to allow one person to check both ends. It must be noted that this unit will only check for pair continuity, pair shorts, crossed wires, and shorts to other pairs. It will not test bandwidth, etc. Operation is fairly basic.

Circuit diagram:

Half of the 4011 quad 2-input NAND gate is an RS flip-flop (IC1a, IC1b) which controls the other half, IC1c & IC1d, operating as a clock oscillator. You can either start and stop the oscillator running by pressing the Start and Stop switches or by virtue of diode D1 connected to pins 12 & 13, use the Stop switch to allow manual clocking of the 4017 counter. The 4017 drives one of eight LEDs and the lines to the RJ45 socket. An output "High" on the 4017 decides which line is under test, and if the circuit is complete, the test LED's current is "sunk" by the 4017 and the LED will light.

If the corresponding test LED on the remote fails to light, then there is a short of that pair in the cable under test. If more than one LED lights, it indicates a short with another pair. A dark test LED on the transmitter indicates that pair is open circuit. "Start" starts the circuit cycling at a rate determined by the 470nF capacitor and 220kO resistor and "Stop/Step" stops cycling, steps through the lines, and when stepped so that no channel LEDs are alight, effectively switches the unit off with a standby drain current of less than a microamp.
Author: Craig Stephen - Copyright: Silicon Chip Electronics
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Ultra Low Drop Linear Voltage Regulator

This circuit is a Mosfet-based linear voltage regulator with a voltage drop of as low as 60mV at 1A. The circuit uses a 15V-0-15V transformer and employs an IRF540 N-channel Mosfet (Q1) to deliver the regulated 12V output. The gate drive voltage required for the Mosfet is generated using a voltage doubler circuit consisting of diodes D1 & D2 and capacitors C1 & C2. To turn the Mosfet fully on, the gate terminal should be around 10V above the source terminal which is connected to the DC output. The voltage doubler feeds this voltage to the gate via resistor R3. IC2, a TL431 adjustable shunt regulator, is used as the error amplifier. It dynamically adjusts the gate voltage to maintain the regulation at the output. With an adequate heatsink for the Mosfet, the circuit can provide up to 3A output at slightly elevated minimum voltage drop.

Circuit diagram:
Ultra low drop linear voltage regulator circuit schematic

Trimpot VR1 is used for fine adjustment of the output voltage. The RC network consisting of R5 and C6 provides error-amplifier compensation. The circuit is provided with short-circuit crowbar protection to guard against an accidental short at the output. This crowbar protection works as follows: under normal working conditions, the voltage across capacitor C5 will be 6.3V and diode D5 will be reverse-biased by the output voltage of 12V. However, during output short-circuit conditions, the output will momentarily drop, causing D5 to conduct. This triggers the MOC3021 Triac optocoupler (IC1) which in turn pulls the gate voltage to ground. This limits the output current. The circuit will remain latched in this state and the input voltage has to be switched off to reset the circuit.
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Internal Resistance Tester For Batteries

This circuit is designed to check the condition of lead-acid and gel cell batteries with capacities greater than 20Ah. It switches a load of about 18A at a rate close to 50Hz so that the internal resistance of the battery can be measured using a digital multimeter across the battery terminals. The measured AC voltage in millivolts divided by 10 (ie, a shift of the decimal point) is approximately equal to the battery's internal resistance in milliohms. As shown, the circuit is quite straightforward and is based on two 555 timer ICs (IC1 & IC2) and power Mosfet Q1. IC1 operates as a monostable timer with a period of 10s.

When switch S1 (Test) is pressed, IC1's pin 3 output goes high for 10s and this enables IC2 which operates as a 50Hz astable oscillator. IC2 in turn drives power Mosfet Q1 which is connected across the load in series with three 0.22W 50W resistors. IC2 then turns off again after 10s - ie, at the end of the monostable timing period. LED1 provides power indication when the circuit is connected to a battery, while LED2 (green) comes on during the test period. The thermostat is not necessary unless the unit is to be used repeatedly (the Jaycar ST-3823 70°C unit is suitable) and you want to protect the output circuit against overheating.

Circuit diagram:
Internal resistance tester for batteries circuit schematic


The power Mosfet does not need cooling but the thermostat and the 0.22W 50W resistors should all be mounted on an aluminium heatsink at least 2mm thick. In practice, the internal resistance of car batteries can vary from about 15mW down to about 3mW. Before testing the battery, check that the electrolyte level is correct and that the voltage across its posts exceeds 12.5V for a nominal 12V battery; ie, close to full charge. That done, switch on the car's headlights and measure the DC voltage between each battery post and its connecting terminal. It should be less than 10mV in both cases; if not, the terminals need cleaning. Once you've done that, you can turn off the headlights, connect the tester and proceed with the internal resistance test. Be sure to connect the multimeter's test probes directly to the battery posts, to read the internal resistance (not the battery terminals).
Author: Victor Erdstein - Copyright: Silicon Chip Electronics
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