Fox flasher MkII – high power 2 LED solar powered beacon

Fox flasher MkII – described an animal deterrent based on an STC 8051 microcontroller and running from a single LiPo cell.

This article describes a further development using a solar cell, shunt regulator, 1S LiPo cell with protection board, and two high power red LEDs.

FF100Above, the unit constructed in a medium size Jiffy box, and a 6V 0.6W PV panel fixed to the top with silicone adhesive. The LDR is fixed to one end with silicone adhesive.

Two SM 1W red LEDs are fitted to opposite sides. They are 120° LEDs, the holes are countersunk to provide for light dispersion and the LEDs clamped to the inside with small brass brackets and heat sink rubber, a little silicone adhesive seals the holes.

FF101The unit is built in a medium size Jiffy box, and a small tab of 2mm ABS is welded to the cover to allow mounting it to a stick and tilting it for optimal solar aspect.

ff201Above, the schematic of the original / basic FoxFlasherII on which this implementation was developed. A very simple circuit with just a handful of electronic components. The LEDs in the basic circuit above are replaced here with two channels of higher current driver and LEDs. BC327s are used for each driver, emitter to +Vcc, base via a 2k2Ω resistor to the MCU output pin, and collector to the output. LEDs are connected with a series 22Ω resistor to limit the current to 100mA in this case, cathode to -ve, anode via the resistor to the BC327 collector. The series resistor should be chosen for the desired current and LED voltage (varies with colour and current).

Whilst the LEDs used can operate up to 350mA, they are quite bright enough at 100mA and the 120° dispersion makes for a very visible beacon.


It is powered directly from a 1200mAh 1S LiPo battery with protection board (ie single cell at 3.6-4.3V) (Lithium battery – 1S protection boards). The MCU shuts down at 3.6V to reduce load on the battery, but the protection board provides primary protection to the LiPo from high and low voltage, and excessive charge or discharge current, the LDR allows the flasher to cease flashing (and so reduce power) during daylight (>30lx).

Since the battery protection board prevents overcharge, a simple 3W 4.7V Zener shunt regulator is used to limit the voltage developed by the PV array to keep the working voltage of the logic to a safe level.


Above, small squares of brass were soldered to the Zener leads to help dissipate heat.


Above, a thermograph of the Zener under load test at 1W, 16° rise above ambient in free air. Maximum dissipation of 0.6W from the PV panel should not raise the temperature more than 10°, though ambient temperature inside the box in the sun may be quite high.

In the enclosure, the hottest part in full sun with fully charged battery (so the Zener is at maximum dissipation is the PV panel, the back of the enclosure is 20° lower in temperature, the finely stippled black ABS finish is quite effective at dissipating heat.

Current is voltage sensitive, but at 4V, power down current is around 0.05mA and operating current is 1.0mA + LED current. For the configured cycle (50ms on, 2000ms off (avg)), average LED current is 100*50/2000=2.5mA. For 12 hours of daylight, total consumption during 24h is around 12*0.05+12*3.5=43mAh giving a battery endurance of 1200/=28 days (ignoring self discharge). Battery capacity is reduced at temperatures below freezing. Budgeting for at least two hours of full sunshine (100mA charge rate) on 66% of days (typical of Winter here), average daily charge of 132mAh is three times daily consumption and with a 28day battery endurance of no charging.

Screenshot - 07_05_16 , 11_46_38

Above is a dump of the EEPROM for inverted output, two outputs, one at a time. In the current implementation, each flash is 50ms, the next LED to flash is selected at random, the wait time between flashes is uniformly randomly distributed between 0s and 4s. The code is more flexible, but simple should be enough to scare foxes, possums, cats etc. There has been a noticeable reduction in animal digging habits near the FoxFlasher.


Above, the prototype under field test. It is optimised for Winter charging, it faces the middle of the available Winter Sun in azimuth, and 10° higher than Winter Solstice altitude (90-Latitude+AxialTilt), so 90-34.5+23.5-10=42°.

Update 2016-06-22

The LDR which was exposed to the weather failed. Though it was set in Silicone sealant, there was leakage under the cap which suggests the cap is not well bonded to the ceramic body.


A new arrangement is on trial. A 3mm disc of Acrylic sheet is glued on the outside of the box over a 6mm hole, and a new LDR glued to the inside of the disc. Time will tell whether the acrylic remains low loss, the glue holds, and the LDR survives.