This eBay item, sold under different names, is a very useful accessory for electronics hobbyists. The gizmo is infact a battery-powered pocket microscope with two white light emitting diodes for object illumination. Only one disadvanatge of this pretty gizmo is that you need to change its internal battery (LR927 x3 button cells) very frequently, which is not cost-effective. So, if you are using the microscope in your worktable, it is better to power from an external power source. Here is a simple yet useful “power supply adapter” circuit built around a few inexpensive components.
First of all, open the battery-room lid and remove all LR927 cells from its metal holder. Next, carefully lengthen the power supply connections by soldering a twin-wire cable/flexible wires from the battery terminal (+/-) points, and drill a 3mm hole at the top of the lid. Through this 3mm hole, you can extend the cable/wires upto the power adapter circuit. The two wire connections terminated into a standard dc plug will be found very handy for linking the microscope to the power adapter.
Since the popular 9V PP3/6F22 battery pack is very convenient for this modification, the power adapter circuit is well- designed for 9V dc operation. Potentiometer (with on/off switch) in the adapter can be used to vary the LED brightness to some extent. After construction, the whole circuit should be encased in a small plastic cabinet.
In this no-frills design, current is controlled using a resistor in series with the LEDs. The potentiometer dynamically adjust the current getting to the LEDs and therefore increasing or decreasing its intensity. This works well for the proposed application. Readers looking for advanced version of an LED dimmer circuit (really not necessary for this project) are requested to refer this link: www.electroschematics.com/2713/led-dimmer-circuit/
Note: The on/off switch of the microscope is a fragile component. That is why an additional power on/off option is included in the adapter. So, we can permanently set the microscope’s on/off switch in “on” mode (no possible damage by frequent fiddling)!