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Circuit Notebook

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.

Automatic bathroom exhaust fan

This circuit starts a bathroom exhaust fan automatically when the shower’s hot water is used. It also includes a push-button switch to allow the fan to be stopped or started at will. The fan runs for a predefined period (nominally four minutes) with reminder beeps at the 2 and 4-minute marks as a water saving initiative.

NTC thermistor TH1 senses water temperature, with an increase in temperature causing a decrease in the voltage applied to the ADC input (pin 6) of IC1. Below about 1.88V, output 2 (pin 5) of the micro is set high, turning on Q1 and energising the relay.

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A second thermistor (TH2) is used in the top leg of the divider to minimise voltage fluctuations due to changes in ambient temperature. The reminder beeps are produced by a piezo buzzer on output 4 (pin 3).

The software is squeezed into 122 bytes of PICAXE memory. The "mainloop" routine reads the ADC input and compares the result with 96 (corresponding to about 1.88V) to determine if the fan should be started. Without a vectored interrupt feature on this chip, the momentary switch (S1) input must be checked repeatedly within the main and timing loops for high (switch pressed) status.

To achieve timing in minutes and to provide appropriate timing points, three loops are nested. To alter the timing period, b2 may be set to 67, 90 or 112 for approx. 3, 4 or 5-minute periods, respectively.

Pressing S1 will stop the fan if it’s already running or start it if not. A 2-minute cooling-off period is included after the fan stops so that it doesn’t start again immediately if the water temperature remains above the set point. A 2-second debounce period is invoked after a high status is detected on the switch input.

Calibration simply involves setting the voltage at the PICAXE’s ADC input (pin 6) close to 2.1V using trimpot VR1. TH1 should be strapped to the shower pipe near the rosette (insulate its leads) and TH2 should be exposed to room temperature. Both thermistors are of the same type (DSE Cat. R 1895).

An electrician should handle all 240VAC mains wiring, while batteries or a plugpack should be used to power the circuit.

W. A. Fitzsimons,
Mount Eliza, Vic. ($45)

'Bathroom Exhaust Fan Controller - 9/2/2005
  readadc 1,b0			'read voltage divider
  if b0 < 96 then timer1		'hot water temperature turns the fan on
  if input3 = 1 then timer1		'momentary switch takes input high
  goto mainloop

  b1 = 0				'reset delay flag
  high 2				'turn fan on
  pause 2000			'debouncing for input 3

  for b4 = 1 to 8			'start timer
  if input3 = 1 then fanoff1
  for b3 = 1 to 243
  if input3 = 1 then fanoff1 
  for b2 = 1 to 90			'67=3min, 90=4min, 112=5min
  if input3 = 1 then fanoff1
  next b2
  next b3
  if b4 = 4 then beep1		'1/2 way reminder
  if b4 = 8 then beep2		'for timer completion

  next b4
  goto fanoff
beep1:				'1 beep on output 4 or turn fan off after delay
  if b1 = 1 then fanoff		'after delay period turn fan off
  pulsout 4,65535			'1 beep on output 4
  goto beep

beep2:				'2 beeps on output 4
  pulsout 4,65535
  pause 1000
  pulsout 4,65535
  goto beep

  if b1 = 1 then timer1		'if delay flag is set then turn fan on
  readadc 1,b0
  if b0 > 96 then fanoff		'no delay required
  if b0 = 96 then fanoff		'no delay required
  b1 = 1				'set delay flag
  low 2				'stop fan
  pause 2000			'debouncing for input 3
  goto timer2

  b1 = 0				'reset delay flag
  low 2				'stop fan
  pause 2000			'debouncing for input 3
  goto mainloop
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Your probing questions answered

This simple but extremely useful idea lets you easily attach various extensions to your multimeter’s probes, thus eliminating the need for a full-length test lead set for each type of probe end.

As shown in the accompanying illustration, a series of whatever ends you normally use (hooks, alligator clips, etc) can be fitted with lengths of flexible cable and terminated with PC board pin sockets (Jaycar Cat. HP-1260). As shown, the joins are insulated with heatshrink tubing.

Your custom designed extensions can be stored in a zip-lock bag with your multimeter so that they don’t get lost or damaged.

Bob Hammond,
Engadine, NSW. ($35)

Automotive LED timing light

A useful timing strobe can be constructed using high-brightness LEDs and a few common components. Ignition pulses from the number 1 cylinder high-tension lead are used to trigger the circuit via a home-made inductive pickup.

Transistors Q1 & Q2 buffer and amplify the pulses from the pickup, which then drive the inputs of three Schmitt-trigger inverters (IC1a, IC1c & IC1f). Each positive pulse at the inverter inputs causes a low pulse at their outputs, forward-biasing D2 and immediately discharging the 6.8nF capacitor.

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When the capacitor is discharged, the inputs of the second bank of three inverters (IC1b, IC1d & IC1e) see a logic low level, so their outputs go high, driving Q3 into conduction and powering the LED array.

After the pulse ends, the IC1a, IC1c & IC1f inverter outputs return high, reverse biasing D2. However, it takes some time for the 6.8nF capacitor to charge to the logic high threshold voltage of the inverters’ inputs, effectively stretching the initial pulse width and lighting the LEDs for the required amount of time.

The pickup can be salvaged from an old Xenon timing light or made up from a "C" type ferrite or powered iron core large enough to fit around a HT lead. Some experimentation will be required to determine the number of turns required to achieve reliable triggering. About 100 turns of light-gauge wire proved sufficient on the prototype.

A cleat is used to close the magnetic path around the lead and is held in place with a large battery clip. Miniature screened microphone cable can be used to connect the pickup to the circuit, to prevent interference from other sources. Refer to the Current Clamp Adapter project in the September 2003 issue of SILICON CHIP for more ideas on how to make the core and clamp assembly.

K. J. Benic,
Forestville, NSW. ($40)

Low-voltage cutout for 12V SLA batteries

This simple circuit protects an SLA battery from over-discharge by disconnecting the load when the terminal voltage drops below a preset level.

In operation, a sample of the battery voltage is derived from the 22kΩ resistor and 20kΩ trimpot divider. This is applied to the non-inverting input (pin 3) of IC1, where it is compared with a reference voltage on the inverting input (pin 2).

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When the sampled battery voltage falls below the reference voltage, IC1’s output (pin 1) swings towards ground, switching Mosfet Q2 off and disconnecting the load from the battery.

The reference voltage is derived from a 4.7V zener diode (ZD1), which is connected to ground via the collector-emitter circuit of Q1 (ie, when Q1 is on). However, when the op amp’s output is driven low, Q1 is switched off, causing the non-inverting input to rise towards the full battery voltage. This greatly reinforces the switching action, latching the circuit in the "off" state until the battery is recharged and the reset switch (S1) pressed.

The Mosfet used for Q2 should be selected to suit the intended application. The circuit could also drive a relay simply by connecting the coil across the "load" terminals. As is usual practice, a diode should be connected across the relay coil to limit back-EMF spikes.

Tim Nuske,
Horsham, Vic. ($35)

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As you can see, we pay good money for each of the "Circuit Notebook" contributions published in SILICON CHIP. But now there's an even better reason to send in your circuit idea: each month, the best contribution published will win a superb Peak Atlas LCR Meter valued at $195.00.

So don't keep that brilliant circuit secret any more: send it to SILICON CHIP and you could be a winner!

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