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I have a project powered by a solar panel which is supposed to charge a battery and then light up an LED only when it's dark outside. I made the initial circuit using an LDR and a potentiometer and although it was a success, it had a serious drawback. The LDR in the circuit board, when placed near the LED, would sense dark and light up the LED, but then it would pick up the light from that same LED and sense it as light and turn the LED off and this would repeat in an endless loop. Since I don't want to place the circuit board with LDR far away from the circuit, I thought I should get rid of it altogether and use the solar panel itself as a light sensor.

So I modified my circuit accordingly but I need help to know whether it would work as I intended. Also I don't know which value resistor I should use for the 2N7000 MOSFET Gate.enter image description here

The LED has an internal resistor so it doesn't need a series resistor in this circuit.

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The circuit shown above will only light if the solar cell is lit with voltage. This means the battery will only light the LED if there is light on the solar panel.

If you want the light to run at night with the battery, you'll need a P-MOSFET on the high side of the battery that turns on when the solar panel voltage is low. You might want to add some hysteresis so the FET is fully on or fully off.

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  • \$\begingroup\$ Oh, ok. What if I use an NPN transistor instead. A TIP 122 should be powerful enough to handle the 2W LED. \$\endgroup\$
    – Kokachi
    Commented Nov 10, 2022 at 19:06
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    \$\begingroup\$ An NPN will aslo only turn on if there is current coming from the cell, which will only happen in the day. \$\endgroup\$
    – Voltage Spike ♦
    Commented Nov 10, 2022 at 19:20
  • \$\begingroup\$ You're right. I came across a circuit that achieves this using a PNP transistor instead. But I couldn't get it to work, don't know why. \$\endgroup\$
    – Kokachi
    Commented Nov 11, 2022 at 16:39
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    \$\begingroup\$ Are you connecting it before or after the diode? Also, the PNP will give you ~0.7V of voltage drop, you should use PMOS. PMOS should go on the high side of the LED \$\endgroup\$
    – Voltage Spike ♦
    Commented Nov 11, 2022 at 17:10
  • \$\begingroup\$ Tried both, didn't work. You are correct about the PMOS, I tested it in the circuit simulator and it worked but unfortunately I don't have any PMOS lying around. I found a makeshift solution that works for now, I don't know if it would cause any problems in the long run. Using an NPN transistor as a Logic Inverter my circuit works as intended at the moment \$\endgroup\$
    – Kokachi
    Commented Nov 11, 2022 at 17:21
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This worked well for me to simply use a solar module as the "dark detector". Here is an explanation.

The first image shows the circuit and the parts used. The second image shows what I measured at the (p channel, high side) MOSFET gate over time, as the day waned, and the solar panel voltage dropped. It works well since 2019 when I put it together, charging the battery during the day. Maximum on-time at full brightness was about 7 hours with that particular LED and battery.

wiring diagram

measured voltages on the high-side MOSFTE gate

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  • \$\begingroup\$ Systembolaget - Hi, Other site members won't understand the background since it's referring to a deleted post, but FYI: (a) Links are allowed - in fact, if you copy material from elsewhere into Stack Exchange, then reference link(s) is/are required. (b) However don't write "link-only" answers. (c) See here where it says "Links to external resources are encouraged, but please add context around the link [...] avoid making it necessary to click on them". We can discuss in chat if you need more info? TY \$\endgroup\$
    – SamGibson ♦
    Commented May 27, 2023 at 20:04

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