Mains zero-crossing recovery

This circuit (Fig.1) is a response to a request from K. R. in Mailbag, May 2006, who asked how a PIC microcontroller could be used to detect the zero crossing points on a 50/60Hz mains signal that is badly distorted at zero crossings.

The input signal is derived from a 9VAC adaptor, which also supplies power to the circuit. Op amp IC1a and associated components form an integrator that also operates as a low-pass filter, greatly reducing the effects of high-frequency noise.

IC1b and Q1 square up the waveform so that the output can be fed to a microcontroller if desired. The output lags the input by 90° (reversing the input connections to IC1b reverses the phase). The microcontroller could be programmed to introduce a delay to produce zero phase synchronisation, as outlined in the accompanying flow chart.

Alternatively, a 0/180° phase out-
put can be achieved with a 4046 (not shown) operated in XOR phase-locked loop mode. The 4046 oscillator may need to be adjusted to give a precise zero-phase lock, if this is a critical requirement.

Note that IC1a’s input is AC-
coupled via a 100nF capacitor to negate the effects of input leakage currents. This has a minor effect on output phase and in most cases can be omitted.

Herman Nacinovich,
Gulgong, NSW. ($40)

Improved vibrating battery tester Many blind and deaf-blind people use portable electronic devices to assist their everyday lives but it is difficult for them to test the batteries used in that equipment. Talking voltmeters are available for the blind but there is no commercially available equivalent usable by deaf-blind persons. This device enables blind, deaf-blind and sighted people to test batteries. It will test AAA, AA, C and D cells, as well as 9V "transistor" batteries. All rechargeable and non-rechargeable cell types are supported. The circuit needs no calibration and is cheaper to build than my design in Circuit Notebook in September 2002. To use the tester, turn potentio-meter VR1 fully counter-clockwise and then connect the battery to be tested to the appropriate set of test terminals. If the battery has any usable charge, the pager motor in the tester will immediately vibrate. VR1 is then slowly rotated in a clockwise direction just far enough to stop the vibration. The position of VR1 then indicates the loaded voltage of the battery on a scale of 1-1.5V (if the battery is connected to the 1.5V test terminals) or 6-9V (if the battery is connected to the 9V test terminals). A regulated +5.1V rail is generated from the battery under test with the aid of zener diode ZD1. For 9V tests, a 150Ω resistor limits the zener current, while diode D2 protects the circuit from reverse polarity battery connection. For 1.5V tests, a blocking oscillator formed by Q1, Q2 and L1 steps up the battery voltage before it is applied to the regulator. This configuration works reliably with inputs down to below 0.9V. The output of the oscillator is rectified by D1 and smoothed by the 33μF capacitor. The circuit has to survive reverse connection of the battery under test. This creates a problem, because the LM393 cannot withstand a voltage more negative than -0.3V at its inputs. Diodes D1 and D2 indirectly protect the non-inverting inputs from negative voltages but series diodes cannot be used to protect the inverting inputs because of the unpredictable voltage drop they introduce. The solution used is to shunt negative voltages at the 1.5V test terminals with diode D3 in conjunction a 1kΩ resistor (R1). D3 limits the voltage at its cathode to about -0.7V, while resistors R2-R4 divide this by three to give no less than -0.23V at the inverting input (pin 2) of IC1a. When the battery is connected the right way around, D3 is reverse-biased and R1-R4 form a voltage divider that applies a quarter of the battery voltage to IC1a’s inverting input. Similarly, D4 and R5-R10 protect the inverting input (pin 6) of IC1b from reverse-connected batteries at the 9V test terminals. However, in this case only 1/24th of the battery voltage appears at IC1b’s inverting input. Battery voltages in the range 1-1.5V at the 1.5V test terminals will therefore produce 0.25-0.375V at the inverting input of IC1a, while battery voltages in the range 6-9V at the 9V test terminals will produce 0.25-0.375V at the inverting input of IC1b. Potentiometer VR1 forms part of a voltage divider used to generate a comparison voltage that is variable over the same 0.25-0.375V range. This is applied to the non-inverting inputs of both IC1a and IC1b. When the sampled battery voltage exceeds this comparison voltage, the respective comparator output swings low, switching on Q3/Q4 to energise the pager motor. The 68Ω resistor in the collector circuit of Q4 ensures that higher battery voltages do not overdrive the motor. When testing an earlier version of this circuit with batteries that have high internal impedance, it was found that when VR1 was advanced to the indicating point, the pager motor slowed down rather than switched off. This occurred due to a rebound in battery voltage at motor switch-off, which in turn caused the circuit to immediately switch the motor back on again. To counteract this effect, a small amount of positive feedback is applied around the comparators when the motor switches off. The feedback is disabled while the motor is running so that the indicating point of VR1 is not affected. This works as follows: when the motor is running, Q5 is conducting and D5 is reverse biased, so the comparison voltage at the non-inverting inputs of the comparators is not affected. If the motor stops running, Q5 switches off and the 2.7MΩ resistor pulls the comparison voltage higher via D5 to ensure that the resulting battery voltage rebound does not restart the motor. Finally, diode D7 prevents reverse breakdown of Q4 in case of reverse battery connection at the 9V terminals. There is no need for a similar diode in the 1.5V part of the circuit because 1.5V is well below the reverse breakdown voltage of Q3. The prototype used "Magtrix" magnetic connectors on short flexible leads as the 1.5V test terminals. These allow the connection of AAA, AA, C and D cells but are arranged so that they cannot be brought closely together enough to connect 9V types. Unfortunately, magnetic connectors cannot be used for the 9V test terminals because some brands of 9V batteries have non-magnetic terminals. A conventional 9V battery snap can be used instead. For blind people, the knob on VR1 should be pointer-shaped (eg, DSE P-7102) so that the degree of rotation can be easily assessed by touch. Andrew Partridge, Kuranda, Qld.

CB radio beeper with selectable tones

This CB radio "roger beep" circuit features 10 selectable tones, is based on a PICAXE-08M micro and is simple enough to be constructed on a small section of prototyping board.

The microphone’s PTT switch must be wired to input3 (P3) of the PICAXE micro. When the switch is closed, the BASIC program immediately sets output0 (P0) high, switching on transistor Q1 and keying the transmitter.

When the PTT switch is released, the PICAXE program plays the currently selected tune, which is injected into the radio’s audio input via a 100nF coupling capacitor. The transistor is then switched off to release the radio’s PTT input and terminate the transmission.

When powered up, no tune is selected and so the radio operates normally. To select a tune, press the "program" switch (S1). This lights the LED for 1.5 seconds, after which the switch can be pressed between 1-10 times to select the tune. After each press, you must pause until the LED goes out (about 0.5s).

Once the program switch has been pressed the desired number of times, a single press of the PTT switch completes the sequence and the selected tone will play. If the switch is pressed more than 10 times, the LED flashes to indicate an error. The LED also functions as a transmit indicator during normal operation.

Unfortunately, the PICAXE program (ROGER_BEEP.BAS) is too large to be reproduced here but it can be downloaded from the SILICON CHIP website.

K.Howell,
Renmark, S.A. ($40)

Inexpensive remote watering system

This remotely controlled watering system is both inexpensive and easy to expand. It is designed to operate in conjunction with a conventional watering timer and allows remote switching between nine zones.

The prototype is used in a bore system, where a deep-well pump must be started and kept running while zones are being changed. This is necessary to minimise cycling and results in maximum pump life.

A standard portable telephone is used as the transmitter and receiver. The system’s range is therefore limited only by the telephone specifications. The prototype uses an Audioline model CDL1A, set to pulse-dial mode via a switch in the side.

Selecting zones from the tele-phone keypad couldn’t be simpler. For the first nine zones, each key number (1-9) corresponds directly to a zone number. If additional zones were added to the basic circuit, "0" would represents zone 10, while further zones are "dialled-in" by simple addition. For example, to select station 15, you’d press "0" and then "5".

Looking now at the circuit, the telephone base station is wired to one input of a hex Schmitt-trigger inverter (IC5a), which functions as a low-pass filter and pulse shaper in conjunction with two 1kΩ resistors, a 10μF capacitor and a second inverter (IC5c).

Glitch-free pulses are fed to the clock inputs of two 74HC164 8-stage shift registers (IC3 & IC4). The A & B inputs of IC4 are permanently pulled high, so the first pulse results in a logic high at output O0 (pin 3). Each additional pulse causes the next successive output to go high. After eight pulses, output O7 (pin 13) goes high and this is propagated to the second shift register (IC4) via its A & B inputs.

The shift register outputs are wired to a collection of 74HC86 exclusive-OR gates (IC6-IC8) in such a way that only one of the 74HC86 outputs can be high at a time. For example, after three clock pulses, outputs O0-O3 of IC4 are high, which results in IC7c’s output going high. The exclusive-OR gates feed a pair of ULN2001A Darlington drivers (IC9 & IC10), which in turn drive relays to switch power to the water solenoids.

If a wrong key is pressed at the remote end and 10 pulses arrive at the shift register inputs, output O1 of IC3 will go high, triggering both 555 timers (IC1 & IC2) via inverter IC5e. The 555s are configured as monostables, so their outputs immediately swing high.

IC2 resets the shift registers, returning all outputs to their initial (low) state. The reset signal is held for about three seconds, which ensures that any number of additional pulses (within reason) above the maximum of nine will be ignored.

In the meantime, IC1 energises one of the water solenoids via diode D2 and the zone #1 driver circuit. This solenoid is held on for about 20 seconds, giving sufficient time for the number to be redialled after the 3-second redial "hold-off" period. This solenoid "hold-on" period is important as it prevents overheating of the pump motor that might otherwise occur without continuous water flow.

The circuit operates from +5V, which is generated by a conventional bridge rectifier (BR1), filter and regulator arrangement. 24VAC for the water solenoids is obtained from the water system timer transformer and is external to this circuit.

Editor’s note: for the "sorry, wrong number" feature to be effective, some form of operator feedback would be required if all of the sprinklers are not visible. Perhaps a siren could also be driven by IC1’s output to alert the operator that a valid sector number must be dialled within 20 seconds!

Francis Egan,
Kew, Vic. ($60)

Contribute And Choose Your Prize As you can see, we pay good money for each of the "Circuit Notebook" items published in SILICON CHIP. But now there are four more reasons to send in your circuit idea. Each month, the best contribution published will entitle the author to choose the prize: an LCR40 LCR meter, a DCA55 Semiconductor Component Analyser, an ESR60 Equivalent Series Resistance Analyser or an SCR100 Thyristor & Triac Analyser, with the compliments of Peak Electronic Design Ltd www.peakelec.co.uk So now you have even more reasons to send that brilliant circuit in. Send it to SILICON CHIP and you could be a winner. You can either email your idea to silicon@siliconchip.com.au or post it to PO Box 139, Collaroy, NSW 2097.