Trouble with Transistor Switch (2n3906)

solar celltransistors

schematic

simulate this circuit – Schematic created using CircuitLab

I'm replicating a circuit found on evilmadscientist.com: the "detecting darkness" http://www.evilmadscientist.com/2008/simple-solar-circuits/

Here is my diagram: http://imgur.com/gallery/q1g62z6/new

the goal is to charge a 9v batt during the day and then at night (when PV is not producing) use a PNP transistor to open the gate and allow the battery to illuminate a bank of 6 LED ( 3sets of 2 ).
the batt-> LED portion does work the PV does produce a current (will illuminate an LED in dim sunlight)

When constructed, the LEDs are always on regardless (as far as I can tell) of the PVs exposure to sun.

I know this isn't so much information, but if someone could help me pin down the problem that would be very helpful.
I sized the components myself so those should be considered suspect.

specs:

resistor preceding 2 LED = 100 ohm 5%
resistor between PV and transistor = 1.5K ohm 5%
diode after PV before batt = 100V 5A small signal Schottky diode
transistor between batt – PV – LED = 2N3906
9V rechargeable batt

Best Answer

There are a number of problems here:

  1. 1N4148 is not a Schottky diode, and certainly can't handle 5 A.

  2. If the panel puts out 6 V max, then it's not going to charge the 9 V battery unless it is very low.

  3. You are overdriving the LEDs at full battery voltage. You say the LEDs drop 3.3 V each. Figure 200 mV for the transistor when fully on, so that leaves 2.2 V across each of R4, R5, and R6, which results in 22 mA.

  4. The turn on/off of the transistor will be "soft". Perhaps this is what you want, but something with a little hysteresis will cause the LEDs to snap on and off.

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