David,
You will have to pick a LDR and stick with it for all 200 units. I suggest the PDV-P8104-ND, stocked by DigiKey at $0.42ea (100pcs). As shown on the data sheet, when illuminated, it has a resistance of between 27KΩ and 60KΩ.
I have designed a 6V Motor driver that will drive two of your motors in-parallel, which has hysteresis (meaning that the 330mA motor driver NFET does not have to be on a heatsink like the Darlington NPN you like would), and has low standby current (300uA), and is adjustable over a range of light levels and variance of the sensitivity of the LDR.
The second most expensive part in this circuit is the NFET. How cheap you can get these depends on what you can work with in how it is packaged. Unfortunately, the really cheap ones are surface mount. This list has ones you can use based on electrical specs, sorted by increasing price. The first through-hole part that would be good in this circuit would be this one. The rest of the parts are not the cost determining factor.
Before I post the circuit, let me know if I am on the right track, cost wise...
You will have to pick a LDR and stick with it for all 200 units. I suggest the PDV-P8104-ND, stocked by DigiKey at $0.42ea (100pcs). As shown on the data sheet, when illuminated, it has a resistance of between 27KΩ and 60KΩ.
I have designed a 6V Motor driver that will drive two of your motors in-parallel, which has hysteresis (meaning that the 330mA motor driver NFET does not have to be on a heatsink like the Darlington NPN you like would), and has low standby current (300uA), and is adjustable over a range of light levels and variance of the sensitivity of the LDR.
The second most expensive part in this circuit is the NFET. How cheap you can get these depends on what you can work with in how it is packaged. Unfortunately, the really cheap ones are surface mount. This list has ones you can use based on electrical specs, sorted by increasing price. The first through-hole part that would be good in this circuit would be this one. The rest of the parts are not the cost determining factor.
Before I post the circuit, let me know if I am on the right track, cost wise...
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