A simplistic approach is to install a miniature light bulb and a 1X5 switch on a 3V power supply (two AA batteries). Generally, a miniature light bulb can be used on a small key light. Each current consumes about 0.1A, as needed. Ignite 1--3. Or with high-brightness focused LEDs, the effect of yellow light 50mA/3V is also good, and it consumes less power than small bulbs.
At present, there is a rechargeable 3V emergency light on the night market. Fully charged for two hours. This light is not very heavy. It can be used for emergency night outings. If it is changed to white high-brightness LED, lighting emergency time It will be very long and can be used for camping.
Due to the limited power supply of the bicycle, the luminaire is required to be more efficient; when using the small bulb, it is better to have a reflective bowl to enhance the front luminosity.
The nighttime lights of mountain bikes are generally difficult to do, and they require good craftsmanship and equipment. Therefore, the general simplification is to add two high-brightness LEDs to the front-end appliances, which is barely enough for emergency.