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Interesting circuit ideas which we have checked but not built and tested. Contributions from
readers are welcome and will be paid for at standard rates.
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Simple logic probe
This simple logic probe has both LEDs on with no signal at the
input but due to the nor gates connected to the probe, indicates correctly when a high or low signal is present.
It also works correctly for pulse trains.
Normally both LEDs are forward biased and therefore on, powered
by the 12V supply. When a logic "high" is present at the probe, IC1a's output goes low sending IC1b's output high. This turns off LED1 but forward-biases (and turns on) LED2.
Conversely, a logic "low" at the probe will send IC1b low,
turning LED1 on and LED2 off.
F. Edwards,
Ardross, WA. ($25)
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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. 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.
www.electronic-circuits-diagrams.com/psimages/powersuppliesckt3.shtml
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12V car battery charger
Unlike many units, this battery charger continuously charges at
maximum current, tapering off only near full battery voltage. In this unit, the full load current of the supply transformer/rectifier section was 4.4A. It tapers off to 4A at 13.5V, 3A at 14.0V, 2A at 14.5V and 0A at 15.0V.
Transistor Q1, diodes D1-D3 and resistor R1 form a simple
constant current source. R1 effectively sets the current through Q1 - the voltage across this resistor plus Q1's emitter-base voltage is equal to the voltage across D1-D3. Assuming 0.7V across each diode and across Q1's base-emitter junction, the current through R1 is approximately 1.4/0.34 = 4.1A.
REG1 ensures that Q1 (and thus the constant current source) is
turned on. When the battery has fully charged, the current through REG1 drops to a very low value and so Q1 turns off (since there is no longer any base-emitter current).
R2 limits the current through REG1. It allows enough current to
flow through the regulator so that Q1 is fully on for battery voltages up to about 13.5V. Decreasing the value of R2 effectively increases the final battery voltage by raising the current cutoff point. Conversely, a diode in series with one of the battery leads will reduce the fully-charged voltage by about 0.7V.
Finally, the MJ1504 requires a good heatsink. The 7815 is
mounted on the same heatsink and will throttle the circuit back if Q1 gets too hot.
Trevor Murray,
East Maitland, NSW. ($30)
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Battery tester for deaf-blind persons
Many blind and deaf-blind persons use portable electronic
devices to assist their everyday lives but it is difficult for them to test the batteries used in this equipment. Talking voltmeters are available but there is no equivalent usable by deaf-blind persons.
This battery tester uses vibration and a user-settable control
to enable blind and deaf-blind persons to test both ordinary and rechargeable AAA, AA, C, and D cells and 9V batteries. For ease of use and maintenance the device is powered by the battery under test.
The design is dominated by the fact that the pager motor will
operate down to only 0.7V. With a 0.3V drop from the switching transistor, a weak cell, at 1.0V, will only just operate the motor. This means that the 1.5V cell sensing circuitry cannot be isolated from the 9V test terminals using steering diodes - they would introduce too great a voltage drop. The solution was to duplicate the level sensing circuitry for each set of test terminals.
On the 1.5V side of the circuit, a resistance network
consisting of two 10kΩ
multi-turn trimpots (VR2 & VR3) and user control VR1a produces an adjustable proportion of the voltage of the cell under test. VR1a selects a division ratio between the low and high limits set by the trimpots. The resistance of VR1a is 10 times larger than the resistance of these trimpots to minimise the interaction between their settings.
The voltage from the resistance network is applied to a combined threshold detector and current amplifier formed by Q1 to Q4 and
associated components. When the threshold (about 0.6V) is exceeded the pager motor is energised, causing the battery tester to vibrate.
In use, VR1 is first set to its fully counter-clockwise
position, then a cell is connected. If the cell's voltage exceeds the 1V low threshold set by the 1.5V LOW trimpot (VR2), the battery tester will vibrate. Rotating VR1 clockwise applies a progressively lower voltage to the threshold detector until a point is reached when the threshold is no longer exceeded and the pager motor switches off. The angle of rotation of VR1 then indicates the
voltage on the battery. VR1 is fitted with a pointer knob to make the angle of rotation easy to feel.
Having the pager motor switch off rather than switch on ensures
that the voltage of the battery is sampled while it is supplying the load of the pager motor. This gives a more accurate indication of the state of the battery than its open-circuit voltage.
To ensure that the user turns VR1 clockwise during the test,
the circuit is designed so that once vibration has ceased, it cannot be made to start again by rotating VR1 counter-clockwise. This also eliminates any possibility of user confusion arising from any hysteresis in the circuit.
This feature is implemented by Q5, which forces the base of Q2
high if Q4 ceases to conduct strongly. A 1μF capacitor between the base and emitter of Q5 forces it off when power is first applied, to give Q4 a chance to conduct. The parallel 1MΩ
resistor discharges the 1μF capacitor when power is removed, to reset the circuit.
To prevent the pager motor being driven through the base-emitter junction of Q5, the base of Q5 is connected to the collector of Q4
via 10kΩ
resistor. Another 10kΩ
resistor is connected in parallel with the pager motor to ensure that Q5 switches on when Q4 switches off.
The 9V test circuit is similar to the 1.5V circuit. A 68Ω
1W
resistor limits the current through the motor to prevent it from being over-driven by the higher voltage. In addition, there is a series diode to protect the 9V circuitry against reverse polarity. A diode is not possible for the 1.5V side of the circuit because it would introduce too great a voltage drop; fortunately, it is also unnecessary since 1.5V is below the reverse breakdown voltage of the transistors used.
The 1μF capacitor across the pager motor smoothes the load provided by the motor so that measurements made by the circuit are consistent from one trial to another. The 1N4001 diode across the pager motor clips any back-EMF generated by the motor.
A D-cell holder and an AA-cell holder connected in parallel
were used for the 1.5V test terminals. The 9V test terminals are the studs from a standard 9V snap screwed to the box.
To calibrate the battery tester, start with VR1 fully
counter-clockwise. First adjust the 1.5V LOW trimpot by turning it fully counter-clockwise, then apply 1.0V to the 1.5V test terminals and turn the trimpot slowly clockwise until vibration just ceases. Now turn VR1 fully clockwise and adjust the 1.5V HIGH trimpot similarly with 1.6V applied to the 1.5V test terminals. There is a small amount of interaction between the low and high settings, so repeat the adjustment of the 1.5V LOW trimpot. Similarly,
calibrate the 9V side of the circuit for a range of 6.0V to 9.6V.
To test a battery, rotate VR1 fully counterclockwise before
connecting the battery to the appropriate set of test terminals (1.5V or 9V). If the device does not vibrate, the battery is completely dead. Otherwise, rotate VR1 slowly clockwise until the device just ceases to vibrate. The position of VR1 then shows the condition of the battery under test.
Andrew Partridge,
Kuranda, Qld.
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Awaken the deaf!
Small alarm clocks and clock-radios often have squeaky piezo
buzzers which may or may not be able to awaken a hard-of-hearing sleeper. Their pitch is up in the region where hearing loss is often greatest.
Larger clocks in a plastic case may have an alarm bell
consisting of a small motor which, as it rotates, flings a tiny washer into contact with a tinny bell. They are better - but still not loud enough for some. Here's my solution to the problem.
You start with a clock with a bell. Unplug it from the mains,
then take the case apart and find the bell motor and the two pins to which it is connected. You may have to ease the motor out to get at the pins.
With power reconnected, adjust the clock settings until the
alarm sounds (ie, motor starts) and carefully measure which of the two pins is positive. Mark this pin. Disconnect power and remove the motor and the bell.
On a small scrap of strip board or even tagstrip, build the
circuit shown. It is just a very simple transistor switch. When the alarm is triggered the transistor is turned on and the electro-mechanical buzzer sounds off.
This is much louder than the bell. The mute switch may be
needed - depending on the original switching method.
The circuit board can be mounted inside the clock case, but the
buzzer should be mounted on the side closest to the user for maximum effect.
Actually two buzzers are much better than one because they
interfere with one another and make quite a din. I found it best to mount the buzzers loosely so that as they buzz they rattle as well.
The 3V battery (2xAA cells in a holder) can usually be squeezed
inside the back of the case.
A.J. Lowe,
Bardon, Qld. ($25).
