Miniature Electronics How Small Can You Go ?

some time ago I get little light (on brown PCB) from one of my friend it was homemade rechargeable signal light with built-in charging circuit, LiIon battery, DIP switch for changing colors on RGB LED and also switching the whole circuit of

but what most attracted my interest was very sensitive darkness sensor

obviously I wanted a schematic of it but he bought it from a guy on some garage sale and he didn't know anything about it even what it was before he brought it to me but he gave it to me because he didn't have any use for it

unfortunately there wasn't any other option only cut it because circuitry was permanently glued with silicone and there wasn't a clear view on the bottom side of sensor PCB because it was sandwich together with charging PCB board

so I Dismantle it, copied schematic and made a first test version with SMD transistors and some resistors which I already had and I also made PCB board in eagle (CAD software)

I always wanted to make some very small circuit without PCB so I call my other friend and ask him if he has SMD resistors which I don't have and on next day I made my first smallest circuit :)

it was a lot of thinking and improvising when I was putting components together but I'm more than happy with the final result if you are interested feel free to watch my steps and try something similar too

Step 1: What You Will Need

1. bucket of patience

2. all needed components

3. "third hand" or some holder for SMD components

4. good tweezer

5. soldering iron with a sharp tip and thin soldering wire with a flux core

6. tongs for cutting pins

7. magnifying glass and other helpfull accessories

Step 2:

1. solder 103 (10k) resistor between base and emitter

2. then connect the base of the transistor to one pin of the photoresistor and you can cut excess length of LDR pin

Step 3:

3. connect small wire to collector of the second transistor (it will be minus for LED later)

4. and solder wire to emitter of the first transistor soldered on photoresistor (this will be later connected with emitter of the second transistor and serve as a ground of finished circuit)

Step 4:

5. connect 102 (1K) resistor between collector of the first transistor and base of second transistor

6. solder 103 (10K) resistor to the collector of first transistor

Step 5:

7. add 472 (4K7) resistor between the end of previously added 10K resistor and unconnected pin of photoresistor

in this point you can cut excess length of the pin on photoresistor but it can serve as grip point for manipulation

8. connect hanging wire added in photo 4 to the emitter of second transistor and solder new wire to the joint between 472 and 103 resistors added in previous steps (photo 6 and 7) it will be VCC wire add LED diode between VCC hub and little wire added in photo 3

I used green LED so I didn't add a resistor for it but if you want to use a different color you will need one 50-100Ω should be enough for other colors (only if you using 3V source)