Lights for Energihuset

Today I designed a solar-cell powered LED lighting for a model of the planned energy education centre Energihuset in Uppsala. The model was built by Nyréns Arkitektkontor and equipped with 108 white light-emitting diodes, series connected in groups of three for a common supply voltage of 12 V. The model will be presented at the Swedish Energiting in Stockholm.

The CIGS thin film solar-cell modules are designed to give a voltage which fits the maximum voltage of 5.5 V for backup-capacitors — a battery of these capacitors will be charged from the solar cells and then provide the power supply for the LEDs. I came up with the following circuit:

Solar cell powered LED light

The central part is a DC-DC converter around a MC34063. This integrated circuit compares the external voltage at its pin #5 with an internal reference voltage of 1.2 V in order to stabilize the output voltage. I connected a light-dependent resistor in parallel to one of the resistors in the voltage divider for this feedback voltage. In the dark the resistance of the LDR is above 300 k&Omega while sinking below 3 kΩ in ambient light conditions. In the circuit this results in an output voltage under illumination which lies below the LEDs’ threshold voltage. In the dark this voltage rises to between 8 V and 12 V, depending on the actual resistance of the LDR and the position of the variable resistor — and the LEDs will light up.

The efficiency of this circuit is certainly not the best. The MC34063 is a simple integrated circuit to work with but it also has a considerable consumption. Even the voltage divider should probably be built with higher resistances — in the current design it draws about 0.5 mA by itself. But the performance should be sufficient for the task at hand: charging a single 1 F capacitor from a 1 W_p solar cell module takes about 90 seconds under a poor halogen bulb and the discharge lets two strings of 3 LEDs each light for about 60 seconds.

The complete system

The circuit board