A recent conversation on Ice In Space forums raised the issue of people chewing through CR2032 batteries because they keep forgetting to turn off their Red-Dot-Finders after a session out in the night sky. A small circuit solved this problem.
Red-Dot-Finders are small finder scopes which use a small red laser dot and a semi transparent mirror to provide a reticule to help point a telescope at a star. For astro-photography they are used mainly at the start of an imaging session where a calibration needs to be performed to ensure the star-map on the computer accurately reflects where the telescope is pointing in the night sky. When using the red-dot-finder it is typically fired up for a few 10s of seconds at the start of an imaging session and then ignored for the rest of the night. By pack-up time the finder is forgotten until the next imaging session when you realise it was running for several days. In the past year this has cost me four CR2032 batteries.
A small modification to the red-dot-finder can do away with the ON/OFF switch in favour of a pushbutton with a circuit that powers the finder for 60 seconds. The schematic for the circuit is shown above, and it is really simple. When S1 (pushbutton) is closed the 3V battery charges C1 which is connected to the gate of MOSFET M1. The MOSFET in this case has a Vgs_threshold of around 1.5V meaning while the gate is above 1.5V the MOSFET is effectively turned ON allowing current to flow and turning on the red-dot-finder.
When the pushbutton S1 is released C1 slowly discharges through R1. The rate of discharge depends on the value of C1 and R1 and follows the formula Vc = Vs.e^(-t/RC). If we want to hit a turn-off voltage of 1.5V in 60 seconds solving for the time constant RC = -t / ln(Vc/Vs) = -60/ ln(1.5/3) = 86. To simplify component selection the time constant of 100s woudl still be in the ballpark of what we are trying to achieve and allows us to select simple values such as 1MΩ resistor and a 100μΩ capacitor. The simulation above shows the result of this selection. Interestingly enough the circuit simulates with the MOSFET switching at 1.5V in just under 60 seconds rather than the 120 seconds which is expected with the component choices. The reason for this is the natural discharge rate of the capacitor (ESR) and the gate leakage of the MOSFET come into play when large resistors are used. The cost and size tradeoffs however make fudging the values a more viable solution than picking a lower R and a higher C.
These values will require tweaking if different time lengths are desired or MOSFETs with different gate threshold voltages are used.
I aimed to make the device as small as possible using SMT components. The entire circuit fitted on the back of a pushbutton which makes it easy to mount in place of the existing switch on the red-dot finder. Another variant of this circuit was designed incorporating its own battery however the resulting circuit is significantly larger and thus wasn’t used. This circuit requires splicing in between the battery and the red-dot finder.
The final circuit worked remarkably well with a press of the push button illuminating the red-dot-finder for almost exactly 60 seconds. Total cost was approximately $4 in components meaning the circuit will pay for itself within a year if it saves me another four CR2032 cells.