Passive LED flasher

LED flashers have been used in a variety of applications. Some flasher units are used in food and hardware stores to attract attention to promotional items and are often attached to boxes distributing product discount coupons. Typically, as shown in the illustration below, these simple units have a red LED on each side of the housing. Inside are the battery and flasher circuitry. The housing also has space for rolls of pre-printed sales coupons. However, the batteries that power the flashing LEDs do need to be replaced every few months. Since flashing always occurs in well-lit stores, there is almost always enough natural and artificial light available to power the flashing unit without the need for any batteries. Instead of batteries, a small solar panel on top of the unit collects enough energy to keep the unit flashing as long as light is available. A medium-sized supercapacitor keeps the available voltage around 5.6 volts. This voltage allows for the use of LEDs of any desired color. I personally prefer ultra-bright green LEDs.

6V solar panels

A typical solar cell, under typical shop fluorescent lighting, produces about 400 microamps of current per square inch of surface area. A 5.6-volt Zener diode limits the voltage to the supercapacitor.

The circuit design below is used to flash two bright, wide-angle LEDs. The energy required for flashing comes from diffused light available in the commercial store. A six-volt solar panel on top of the flasher assembly converts some of the light hitting the solar panel into electrical energy, charging a supercapacitor. The supercapacitor provides the peak current required to flash the two LEDs. A constant current LED driver is used. The two LEDs are connected in parallel. Each receives approximately 10 mA of current for 20 milliseconds. The flashing rate is once per second. Therefore, the average current is approximately 400 microamps.