v 20081231 1
C 40000 40000 0 0 0 title-B.sym
C 43400 48500 1 0 0 photo-resistor-1.sym
{
T 44100 49200 5 10 0 0 0 0 1
device=PHOTORESISTOR
T 43600 49200 5 10 1 1 0 0 1
refdes=R1
}
T 44200 48900 9 10 1 0 0 0 2
CdS
photocell
C 42700 48600 1 0 0 battery-1.sym
{
T 43000 49500 5 10 0 0 0 0 1
device=BATTERY
T 43000 49100 5 10 1 1 0 0 1
refdes=B2
T 43000 49900 5 10 0 0 0 0 1
symversion=0.1
}
C 46100 48300 1 180 0 resistor-1.sym
{
T 45800 47900 5 10 0 0 180 0 1
device=RESISTOR
T 45900 48000 5 10 1 1 180 0 1
refdes=R2
}
T 48600 45400 9 14 1 0 0 0 4
Circuit Output:
Dark Output < 3 mV
Room Light ~ 2.2 V
Saturation output at ~ (Vb - 1) Volts
T 42600 45400 9 14 1 0 0 0 4
Photocell:
Dark R > 2 MOhms
Room Light R ~ 20 kOhms
Bright Light (near 40 W bulb) ~ 100 Ohms
T 42200 48400 9 10 1 0 0 0 1
Vb
T 50100 40700 9 16 1 0 0 0 1
Light Meter
T 53900 40400 9 10 1 0 0 0 1
2009-12-26
T 54000 40100 9 10 1 0 0 0 1
K. Myneni
T 50100 40100 9 10 1 0 0 0 1
1
T 51600 40100 9 10 1 0 0 0 1
1
T 50000 40400 9 10 1 0 0 0 1
light-meter.sch
C 42000 48600 1 0 0 battery-1.sym
{
T 42300 49500 5 10 0 0 0 0 1
device=BATTERY
T 42300 49100 5 10 1 1 0 0 1
refdes=B1
T 42300 49900 5 10 0 0 0 0 1
symversion=0.1
}
N 45200 48200 45100 48200 4
N 42700 49400 44900 49400 4
T 42900 48400 9 10 1 0 0 0 1
Vb
N 44300 48800 45100 48800 4
N 45600 49400 45600 50000 4
N 45600 50000 42000 50000 4
N 42000 50000 42000 48800 4
N 45100 49200 44900 49200 4
N 44900 49200 44900 49400 4
N 46100 49000 46800 49000 4
N 43400 48800 43400 48400 4
N 43400 48400 45600 48400 4
N 45600 48400 45600 48600 4
N 42700 47200 42700 49400 4
T 51700 46800 9 14 1 0 0 0 10
Parts:

R1  Cadmium Sulfide Photocell
R2  4.7 kOhm
R3  15  kOhm 
C1  0.1 uF
U1 Op-Amp (LM741 or equivalent)
B1, B2  9V Transistor Batteries
Other: 9V battery leads, small breadboard 
or protoboard, analog/digital voltmeter
T 46900 49900 9 10 1 0 0 0 1
Vout = Vb x (R2/R1)
C 45100 48600 1 0 0 lm741-1.sym
{
T 45725 49550 5 8 0 0 0 0 1
device=LM741
T 45300 49500 5 10 1 1 0 0 1
refdes=U1
}
C 45200 47300 1 0 0 capacitor-1.sym
{
T 45400 48000 5 10 0 0 0 0 1
device=CAPACITOR
T 45300 47600 5 10 1 1 0 0 1
refdes=C1
T 45400 48200 5 10 0 0 0 0 1
symversion=0.1
}
N 45100 47500 45100 48800 4
N 45100 47500 45200 47500 4
N 46100 47500 46100 49000 4
C 46800 48700 1 0 0 voltmeter-1.sym
{
T 46900 49550 5 10 0 0 0 0 1
device=VOLTAGE_SOURCE
T 46900 49400 5 10 1 1 0 0 1
refdes=Voltmeter
}
C 47800 48100 1 90 0 resistor-1.sym
{
T 47400 48400 5 10 0 0 90 0 1
device=RESISTOR
T 47500 48300 5 10 1 1 90 0 1
refdes=R3
}
N 42700 47200 47700 47200 4
N 47700 47200 47700 48100 4
T 41000 41500 9 14 1 0 0 0 11
Notes:

1. If photocell resistance is substantially different than noted above, R2 must be adjusted to give 
    corresponding circuit output.

2. The circuit components are chosen to provide useful metering for indoor lighting. If sensitivity to 
    a narrower or wider range of light level is desired, adjust R2 accordingly.

3. The output should be fairly linear over its operating range, except near the saturation level of
    the op-amp. Typical CdS photocells have slow response (tens of milli-seconds), so such a 
    detector is not useful for detecting modulated light except at very low frequencies.