Forrest M. Mims III: Booster Light for Hard Drive Activity LED


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The hard drive activity LED of a desktop computer can be difficult to see when the computer is placed under or adjacent to a desk. Sometimes it's helpful to know that your hard drive is active, and this project will let you know by boosting the dim glow of the HDD activity LED into a much brighter light.

How It Works

The circuit uses a small cadmium sulfide photoresistor to detect the glow of a hard drive activity LED. As shown in Fig. 1, the photoresistor and R1 form a voltage divider connected to the inverting input of a LTC1050 op amp connected as a comparator. Trimmer pot R2 forms a second voltage divider that is connected to the non-inverting input of the LTC1050. In operation, the photoresistor is shielded from light and R2 is adjusted until the output LED just switches off. The LED will then switch on when the photoresistor is illuminated by even a very weak source of light.

Circuit for harddrive
Figure 1. Circuit for hard drive activity LED booster light.

R2 controls the sensitivity of the circuit by controlling the voltage applied to pin 3 of the op amp.

Order and build your own Booster Light (P/N 2114298)

Parts You Will Need

The following parts were used to assemble a breadboard version of the circuit (Jameco catalog numbers in parentheses):

IC1 – LTC1050 op amp
PC1 – Cadmium sulfide photoresistor or similar
R1 – 1M resistor
R2 – 1M trimmer pot
R3 – 1K resistor
LED – any super bright red or green LED

Miscellaneous: Perforated prototype board, 9-volt battery, battery connector clip, double-sided tape or 9-volt battery holder and wire jumpers.

Note: While the components listed above were used for the prototype, substitutions can be easily made.

Prepare the Board and Install the Components

The circuit was assembled on a solderless breadboard and tested. When the circuit was operating properly, the components were transferred to a 1 x 3.5 inch (2.5 x 9 cm) board cut from a perforated prototype board and soldered in place.

If you want to make a permanent version of the circuit, you can follow your own parts layout, and you might consider installing the circuit in a small enclosure. Or you can simply copy the layout I used shown in Fig. 2 to make a trial version of the circuit. The one shown in Fig. 2 can be easily attached to a computer using various kinds of putty or tape designed for wall hangings.

Parts layout

Figure 2. Parts layout for an assembled version of the hard drive LED booster. The LED is glowing because it is exposed to the light required to make the photograph.

You can follow the steps below to duplicate the prototype circuit shown in Fig. 2 or you can go on your own.

1. The prototype board shown in Fig. 2 was cut from one end of a board along row 49 and the cut edge was filed smooth. This provides a 1 x 3.5 inch (2.5 x 9 cm) board with an appropriate foil pattern on the back side for the parts layout shown in Fig. 2.

2. Cutting the board from the end of the source board provides an alphanumeric label (e.g., E57, etc.) for each hole. These labels are used in the following steps. If the board you use doesn't have labels, you can proceed by comparing each step with the circuit diagram and the parts layout in Fig. 3 below.

3. With the bottom edge of the board being row 58, insert the LTC1050 op amp IC into the top side of the board (without the foil pattern) so that pin 1 is in hole D56 and pin 5 is in hole G53. Bend pins 1 and 5 slightly outward to secure the IC in place.

Components in the circuit

Figure 3. Close up view of the components in the circuit. The LED is glowing

4. Insert trimmer R2 in the board as shown in Fig. 3. The center pin should be inserted into hole J58. Bend the pins slightly outward to hold R2 in place.

5. Insert R1 in holes E57and G57 as shown in Fig. 3. Note that its leads bridge pins 2 and 4 of the LTC1050.

6. Insert R3 as shown in Fig. 3 between holes C52 and E52. Note that one lead goes to pin 7 of the LTC1050.

7. Check to make sure all components are properly positioned. Then use a low-wattage soldering iron with a well-tinned, small tip to solder all the pins and leads of the above components in place. Be careful to avoid applying too much solder.

Caution: Be sure to work in a well ventilated room when using lead solder.

8. Trim the leads of the resistors so that 1/4 inch (6 mm) or so remains for additional connections.

Caution: Wear eye protection when using a wire cutter to trim leads and pins.

9. Insert the LED between holes C51 and F51 as shown in Fig. 3. Important: The lead closest to the flat spot at the base of the LED must be inserted into hole F51. Solder the leads in place and clip off the excess.

10. Use a hobby knife or small drill to form a 1/8 inch (3 mm) hole in the board at hole H52. This hole is for the battery connector leads.

11. Insert the battery connector leads through the top side of the board at hole H52 and tie them into a knot on the foil side of the board. Leave at least 3 inches (75 mm) of wire length on the top side of the board.

12. Flip the board over and trim the red and black leads to about 1 inch (2.5 cm). Carefully remove about 3/16 inch (5 mm) from the ends of the red and black battery clip leads.

13. Solder the red lead to the lead from R3 at hole E52.

14. Solder the black lead to the pin from trimmer R2 at hole K57.

15. Refer to Fig. 3 above and insert about 1/4 inch (6 mm) of the ends of the leads from PC1 into holes A54 and A56. Be sure to install PC1 so that its sensitive surface faces AWAY from the front side of the circuit board as shown in Fig. 3. (PC1 is not polarized.) Solder the leads in place and clip off the leads, leaving 1/8 inch (3 mm) for future connections. Bend the leads from PC1 so that the photoresistor is extended away from the circuit board as shown in Fig. 3.

16. Solder a length of wrapping or other small gauge insulated wire between the red battery lead at hole E52 and the pin from trimmer R2 at hole I57.

17. Solder a length of wrapping or other small gauge insulated wire between the red battery lead at hole E52 and the lead from PC1 at hole A54.

18. Solder a length of wrapping or other small gauge insulated wire between the lead from PC1 at hole A56 and the lead from R1 at hole E57.

19. Solder a length of wrapping or other small gauge insulated wire between the pin from trimmer R2 at hole K57 and hole G58.

20. Solder a length of wrapping or other small gauge insulated wire between the center pin of R2 at hole J58 and hole F58.

21. Carefully check the circuit for any wiring errors. Be sure that exposed component leads and wires do not touch one another. Remove any solder bridges between foil traces using braided solder remover.

22. After the circuit has been checked and any problems corrected, attach the battery to the board using double-sided tape or a 9-volt battery holder (see Parts List).

Testing the Circuit

Rotate R2's shaft to its midpoint. Place a piece of black electrical tape over the sensitive surface of PC1.

Connect the battery clip to the 9-volt battery. With the circuit is a darkened location, rotate R2's shaft until the LED just switches from on to off. Use black electrical tape to attach PC1 over the hard drive activity LED of a computer in idle mode. If the LED on your circuit glows while the hard drive is idling, darken the room lights or apply a second layer of black tape over PC1. If this doesn't work, rotate R2's shaft slightly until the LED switches off. Now put the hard drive to work by viewing files or photos on the computer. The LED on your circuit should glow brightly as the hard drive accesses the files.

Going Further

The circuit shown here is only one way to exploit the hard drive activity LED on a computer. You can also use the activity LED to trigger a tone generator if you prefer to hear what's going on with your hard drive.

About Forrest M. Mims III