Circuit to detect the flash rate of the Solar PV Generation Meter

There will be several circuits in this series, all based on this first simple detector design.

I intend finally to combine the Owl Switch Circuit with a development of this Simple Flash Rate Detector to produce one of two ‘stand alone’ designs which can be used to control the use of the excess power generated by a solar PV system.

With the intention of combining this circuit with other circuits I have included a 5V voltage regulator (U1) so that any supply voltage between 7V and 12V DC may be used. If you do not wish to expand the circuit further and you have a stable 5V DC supply available, U1 and C2 can be omitted.

Most Generation meters have an LED indicator which flashes whenever energy is being generated by the solar panels. The rate at which the LED flashes is determined by the rate of generation. Usually the rate will be 1,000 flashes per hour when 1kW is generated. This increases to 2,000 flashes per hour for 2kW etc. The circuit I describe includes a ‘Power Select Control’ (a variable resistor with a control knob) which has a selection range covering 1kW to 4kW.

The circuit uses a ‘phototransistor’ type BPW85 to detect the flashes. The phototransistor is sensitive to light and will react when it detects the light from the LED on the generation meter. In my version of the circuit I have connected the two wires from the phototransistor to a short length of wire – about 30cm long. I attached the phototransistor to the generation meter using a small piece of Blu-Tack. The transistor is attached so that the domed end of it faces directly onto the surface of the LED on the generation meter.

The IC which detects the flash rate is U2, a 74HC123 retriggerable monostable. A monostable is a device which produces a pulse output whenever it is triggered. The time length of the pulse depends upon the capacitor C3 and the combined resistance of R1 plus the Power Select Control. If the Power Select Control is set so that the pulse length is 1 second, the output on U2 pin 13 will jump to 5V for one second when the input on pin 1 is triggered by the phototransistor. The 74HC123 is a retriggerable monostable. This means that if the next trigger on pin 1 occurs before the one second has timed out, the output will remain at 5V for a further 1 second. This means that if the flash rate is faster than one second, the output on pin 13 will remain at 5V, but if the flash rate is slower than one second, the output on pin 13 will fall to 0V before the next trigger arrives. If the generation meter flashes occur at 1 second intervals, it means that there will be 3600 flashes per hour – so 3.6kW is being generated.

The Power Select Control can be varied to change the pulse length from 3.6 seconds (=1kW) to 0.9 seconds (4kW).

The output pulse on pin 13 charges capacitor C4 through the resistor R3. It takes around 10 seconds to charge the capacitor. Should the output on pin 13 fall back to zero (flash rate is too slow) the capacitor C4 is quickly discharged through the diode D1 and the charging process has to begin again. In this way, the capacitor C4 will only charge up if the flash rate of the LED on the generation meter is faster than the level set by the Power Select Control.

The integrated circuit 74HC14 (U3) is used to turn on an indicator light (LED1) and also a Solid State Relay (SSR) connected to the output of Q1.

It is interesting how one can get carried away with trying to emulate another’s circuit. My first prototype was built to be an alternative to the Maplin kit http://www.maplin.co.uk/light-activated-switch-kit-528796 as used in the circuit described by forum member energysavingexp in MSE forums. The Maplin circuit uses a relay on the output to drive an SSR. The Maplin’s relay is connected so as to switch a low voltage DC signal to the SSR control pins. My original prototype copied this idea. It was only when trying to find a suitable multi-sourced relay that I realised that the relay on the output of my circuit was not necessary. The transistor Q1 is quite capable of driving the SSR directly.

In this simple circuit, no use is made of the Reset input, pin 3 of U2. Also not used are four sections of U3 and one section of U2.