Miniature light bulb tester/evaluator.

New! Finally the long awaited plans for a miniature bulb tester!

Here is a project that's perfect for anyone who restores light sets - a light bulb tester designed to test and identify the miniature bulbs used on series wired Christmas light sets. It has two D.C. power supplies designed for this purpose. Supply A, based on the LM723 analog voltage regulator, is selectable to 2.5, 3.5, and 6.0 volts. Supply B, which uses the LM317 analog regulator, is continuously variable from 1.2 to 13 volts, which includes just about every bulb voltage used. This tester also has a meter to measure the current draw of a bulb, which is very important with series wired sets.

Tester Prototype

Breadboard version of tester. This looks like something out of Thomas Edison's laboratory, but it works. The multimeter shows supply A set for 6 volts output.

Circuit description and operation.

110 volts A.C. goes to fuse F1, then SPST switch SW1, and is input to transformer T1. T1 steps the voltage down to 12 volts A.C. Bridge rectifier module CR1 converts the A.C. to D.C. which is filtered by C1. R9 and  LED pilot light DS1 show power on status and also serve to drain the charge on C1 when power is turned off.
Supply B uses U1, an LM723 analog voltage regulator. SW2, R1,R2,R3 and R4 set U1  to output 2.5, 3.5, and 6.0 volts. Q1 is an NPN current boosting transistor configured in emitter follower mode. R5 sets the supply to current limit to 0.5 amps, enough to test most mini bulbs. Current limiting is important because if the bulb you are testing is burned out, then it's shunt will likely be tripped, and when that happens the bulb becomes a dead short.
Supply A uses U2, an LM317T adjustable regulator which needs only R6 and potentiometer R7 to complete the circuit. U2 is current limited to 1.5 amps by design.
Meter M1 is a 0 to 200 millivolt digital panel meter. Current sense resistor R10 scales the meter to read 0 to 2 amps. The meter is placed after an SPDT center off switch, so it will read current from supply A or B.
T2, D5 - 8, U3, R11, C3 and C4 form a low ripple 9 volt supply to power the digital panel meter. If you want to simplify things, you can power the meter with a 9 volt transistor battery and switch. Note that the 9 volt supply for the meter must be a "floating" supply isolated from the lamp test supply. IE: no common ground!
Note: 2 separate transformers are used only because it was not possible to find a transformer with 2 secondaries.

Schematic Diagram of tester.

Schematic page 1

Page 1

Schematic page 2

Page 2

Parts list for Miniature Bulb tester. Three sources are listed with links to their home pages at the top.
Parts with an asterisk* are critical values and should not be substituted.

Part reference Part type NTE Electronics Radio Shack Jameco Electronics
U1 LM723 regulator NTE923D   24467
U2 LM317T regulator NTE956 276-1778 23579
U3 LM78L09 regulator NTE1902  
Q1 TIP29 transistor NTE291  
R1* 8.2 K resistor QW282BR  
R2*, R4* 4.7 K resistor QW1475BR  
R3* 820 ohm resistor QW0825BR  
R5* 1.0 ohm resistor, 1/2 W HW1D0  
R6 2.0 K resistor QW220BR  
R7* 1.0 K resistor QW210BR  
R8* 10 K ohm pot 271-1715
R9 750 ohm resistor QW0750BR  
R10* 0.1 ohm resistor, 1/2 W HWD10  
R11 470K ohm 1/4 watt QWCC447    
CR1- CR4 Bridge Rectifier   276-1146 178183
CR5 - CR8 Bridge Rectifier   276-268 102980
C1 2200 uF 35 V capacitor NEH2200M35 272-1020  
C2 100 pF capacitor 89110 272-123  
C3 220 uF 35 V capacitor NEV220M35  
C4 4.7 uf 16V capacitor TD4.7M16  
T1 12V,1.2A transformer   273-1352  
T2 12.6V 300ma transformer   273-1385  
SW1 SPST Toggle switch    
SW2 Rotary switch 3 position   G3SR024-R
SW3 SPDT center off switch   21910
M1 3.5 digit panel meter   175951
DS1 Green LED 30038  
Project box 7x5x3   270-1807  
F1 Fuse, 1/2 amp   270-1003 (4 pack)  
Fuse holder Block type   270-739  
Fuse holder (alt) Panel type   270-367  

Construction - The circuitry is mounted on perforated board typically used for prototyping. Q1 and U2 are mounted on a heat sink and also must be insulated from each other. As you can see in the photo below, the transformers, circuit board and heat sink are in the bottom of the box, while the meter, switches, indicator light and test connectors are mounted on the cover.
Note: the only reason 2 separate transformers are used is because it was not possible to find a transformer with 2 secondaries.

Bulb tester opened.

Tester shown with cover removed. Click photo to see larger view.

Operation - To test a bulb, first install it into the appropriate socket depending on base size it will be 1/4 inch or 5 millimeter. Use the binding posts if a bulb just has bare wires. It may be necessary to scrape the oxide from the wires to insure good contact.
Now adjust the output voltage to the lowest setting and turn on the output. The bulb should light up and the current meter will show a current draw. Now increase the voltage until the light output looks right. Watch the base area where there is no paint coating.  Look for the light to turn yellow to white at that point you will have the correct voltage rating for the bulb. Now observe the current. You now know the bulbs voltage and current rating.
If the bulb is burned out, there are two possibilities. If the filament and shunt are open there will be no current draw. Make sure you have good contact in the socket if you see no current.
The second possibility is for the shunt to be activated. Then there will be a high current draw because the shunt is a short circuit across the bulb.


Footnotes - These plans are considered copyrighted material. You may: Print the plans and use them to construct the tester for your own use, or link to this page from another website.
You may not: Post photos from this page to other photo sharing websites, or sell the plans.