Solar hot water panel differential pump controller

This circuit optimises the circulation of heated water from solar hot water panels to a storage cylinder. It achieves this by controlling a 12V DC pump, which is switched on at a preset temperature differential of 8°C and off at about 4°C.

This method of control has distinct advantages over some systems that run the pump until the differential approaches 0°C. In such systems, the pump typically runs whenever the sun shines.

A small 10W solar panel charging a 12V SLA battery is sufficient to run the controller. Most commercial designs use 230VAC pumps, which of course don’t work when there is a power outage or there is no AC power at the site.

Operation

Temperature sensors TS1 & TS2 are positioned to measure the highest and lowest water temperatures, with one at the panel outlet and the other at the base of the storage cylinder. The difference between the sensor outputs is amplified by op amp IC1d, which is configured for a voltage gain of about 47. As the sensors produce 10mV/°C, a difference of 8°C will produce about 3.76V at the op amp’s output (pin 14).

The output from IC1d is fed into the non-inverting input (pin 10) of a second op amp stage (IC1c), which is wired as a voltage comparator. The op amp’s inverting input (pin 9) is tied to a reference voltage, which can be varied by trimpot VR3.

When the voltage from IC1d exceeds the reference voltage, the output of the comparator (pin 8) swings towards the positive rail. A 10MW resistor feeds a small portion of the output signal back to the non-inverting input, adding some hysteresis to the circuit to ensure positive switching action.

A third op amp stage (IC1b) acts as a unity-gain buffer. When the comparator’s output goes high, the buffer stage switches the Mosfet (Q1) on, which in turn energises the pump motor. Mosfet Q1’s low drain-source on-state resistance means that in most cases, it won’t need to be mounted on a heatsink.

The prototype uses a Davies Craig EBP 12V magnetic drive pump, which draws about 1A when running and is suitable for low-pressure hot water systems only (don’t use it for mains-pressure systems as it may burst!). For mains-pressure systems, the author suggests the SID 10 range of brass-body magnetic drive pumps from Ivan Labs USA.

Setup

Each LM335 temperature sensor and its associated trimpot is glued to a small copper strip using high-temperature epoxy. It is then waterproofed with silicon sealant and encapsulated in heatshrink tubing. Standard twin-core shielded microphone cable can be used for the connection to the circuit board.

Before sealing the two units, adjust their trimpots to get 2.98V at 25°C [(ambient temperature x .01) + 2.73V] between the "+" and "-" terminals. When both have been adjusted, clamp them together and allow their temperatures to stabilise for a few minutes.

Next, measure the output voltage from the differential amplifier (IC1d), which should be close to 0V. If not, tweak one of the pots until it is. Separate the two and warm the panel sensor (TS1), monitoring the output of IC1d. You should see a marked increase in voltage, remembering that an 8°C difference between the sensors should give an output of about 3.76V.

The pump switch-on point is set by VR3 and can be adjusted over a practical range of about 4-10°C differential (1.88-4.70V). Adjust VR3 to get about 3.8V on pin 9 of IC1c as a starting point. If set too low and the panels are located far from the cylinder, much of the heat will be lost in the copper connecting pipes. On the other hand, if set too high and the weather is mostly cloudy, then the pump will not switch on very often, as the panels will not get hot enough.

For best results, use copper pipes for the panel plumbing and insulate them with tubes of closed-cell foam. As the pipes cool down between pump operations, small diameter pipes of 15mm are more efficient than larger sizes as they contain less static water.

In practice, the pump in the author’s setup switches on for about 30 seconds every 4-5 minutes. As the Davies pump shifts 13 litres/minute, it displaces the heated water from a single panel in about 14 seconds. There is a thermal lag in the sensor readings, so after the pump stops, the temperature difference will keep decreasing for 40 seconds or so as the panel sensor cools down and the cylinder sensor heats up.

Mike Scaife, Porirua City,

Wellington, NZ. ($50)

The "Mystery Swinger"

Here’s a natty mobile with flashing lights and no visual drive mechanism. To make the base part of the swinger, remove the drive module and battery holder from a discarded pendulum quartz clock. Install it under the lid of a plastic box with the coil pointing up and bring the battery negative terminal and output (transistor collector) leads out for use later.

Next, make an arch of wood or plastic centred over the plastic box and from the top centre of this suspend a swinger, positioned so that it just clears the box. The author used a light coil spring about 9cm long for the job, hung by a thread to give very free motion. A small disc magnet (saved from the pendulum clock just dismantled) is glued to the bottom of the swinger.

Now when the batteries are installed, the swinger will swing randomly as it is not constrained as it is in an ordinary pendulum. Its source of power is invisible – the "mystery"!

Connect the leads from this device to the circuit shown and you have a set of 10 LEDs flashing sequentially in time with the swings. The circuit is just a standard 4017 ring counter (IC1) with the drive provided by Q1 which is turned on and off by the pulses from the collector of the transistor in the drive module.

The 470nF capacitor is necessary to prevent spurious oscillations. The LEDs are arranged around the arch and may be in strict order or wired to appear random. If you prefer LEDs of various colours, you’ll need a suitable current-limiting resistor in each cathode line, rather than the common 470W unit used here.

Try holding the swinger steady over the drive module – if the spring is very light, it will dance up and down (well, vibrate vertically) when you let go and set the LEDs flashing merrily.

A. J. Lowe,

Bardon, Qld. ($35)

Clap-controlled switch

This circuit can switch two or more devices on and off in response to a series of rapid handclaps. The claps are picked up by an electret microphone and amplified by a 741 op amp (IC1).

IC1 is configured as an inverting amplifier, with its gain and hence the sensitivity of the circuit adjustable via trimpot VR1.
Its output is then fed into a pulse-shaping stage based on a 555 timer (IC2). The 555 is configured as a monostable multivibrator, with its trigger input (pin 2) normally biased to about 0.4V_CC by the 150kW and 100kW resistors.

When a loud enough sound is detected, pin 2 will be pulled below 1/3V_CC, triggering the monostable. The output (pin 3) will immediately swing high, causing transistor Q1 to conduct. The result is a 250ms low-going pulse at the collector of Q1.

The output from the pulse-shaper stage is fed into a missing-pulse detector based on a second 555 timer (IC3), which is also configured as a monostable multivibrator. However, this monostable circuit differs from the first because it includes a "retrigger" function.

Retriggering is made possible by transistor Q2, which acts to rapidly discharge the 10mF timing capacitor should a pulse arrive when the timer is already running. This means that once triggered, IC3’s output will remain high as long as additional trigger pulses are received within its set timing period.

Pulses from Q1’s collector are also applied to the clock (CP0) input of a 4017 decade counter (IC4). In the initial state, output 0 (pin 3) of the counter is high, illuminating LED1. The first pulse advances the count and lights LED2, indicating that the circuit is active and ready to receive a handclap "command".

Each time another pulse (clap) is received before IC3’s timing period expires, the process repeats, incrementing the counter by one.
When IC3’s timing period is allowed to expire (ie, no claps have been detected for 750ms), the output (pin 3) will go low, turning off transistors Q3 & Q4.

The rising voltage on the collector of transistor Q3 clocks two J-K flipflops (IC5a & IC5b), with the result at their Q outputs dependent on the state of the O2-O5 counter outputs. Considering all the possible logic states of a J-K flipflop, the "commands" will therefore be:

2 claps: turn Device 1 on

3 claps: turn Device 1 off

4 claps: turn Device 2 on

5 claps: turn Device 2 off

Finally, the rising voltage on the collector of Q4 resets the counter, ready for the next clap sequence. Note the addition of a 10nF capacitor between the reset input (pin 15) and ground, which in conjunction with the 10kW resistor adds a short delay to the reset signal. This ensures that the counter is not reset until after the J-K flipflops have been clocked.

If desired, the circuit could easily be expanded by adding more flip-flops and counters.

Li-Wen Yip,

Douglas, Qld.

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