HDD Activity Meter
This circuit is designed as a replacement for the standard HDD activity led and offers an indication of the amount of activity by lighting a row of leds. The higher the HDD activity, the more leds light. It does not give a reading of actual Mb/s, just how hard the drive is working.
The circuit is based around the "Classic" Knight Rider design and differs only with R3 being fitted in it's alternate position, the addition of IC4 and R6, and VR1 is replaced with a wire link.
The opto-isolator, IC4, connects to the HDD led header on the motherboard and serves to isolate the circuit from the rest of the system whilst ensuring compatibility with just about every motherboard that has a HDD led. As the HDD reads and writes it produces a stream of pulses which are fed to the circuit via the opto-isolator. These pulses are fed into IC1b, which buffers and shapes these pulses before feeding them into IC1a. IC1a is an integrator whose job it is to "smooth" these pulses into a voltage level. This voltage is reasonably proportional to the frequency and duration of the pulses generated by the HDDs activity. Hence, the busier the drive, the higher the voltage. This voltage is fed from the output of IC1a into IC2 which is an led bargraph voltmeter. So in a nutshell, the busier the drive, the more leds light up. Similar to the previous circuits, the display can be operated in either DOT or BAR mode and has 10 and 20 led variations.
VR3 sets the "zero" and is adjusted so that all the leds are just extinguished with no HDD activity. VR2 is the "span" and is adjusted so that all the leds are lit during heavy or constant drive activity. This span adjustment is somewhat subjective and is found by trial and error during HDD activity.
Parts List with order codes for UK and US suppliers
| Resistors all 0.6W 1% Metal Film | |||
| R1,2,3 = 47K | 62-2178 | M47K | QW347 |
| R4 = 100K | 62-2194 | M100K | QW410 |
| R5 = 1K | 62-2098 | M1K0 | QW210 |
| R6 = 3K3 | 62-2122 | M3K3 | QW233 |
| R7,8 = 3K3 | 62-2122 | M3K3 | QW233 |
| Presets all 22 turn or single turn | |||
| VR1 = 220K multiturn (recommended) | 68-0432 | UH28F | 38-124 |
| single turn (cheap option) | 67-0555 | WR45Y | 38-214 |
| VR2 = 10K multiturn (recommended) | 68-0422 | UH25C | 38-237 |
| single turn (cheap option) | 67-0530 | WR42V | 38-120 |
| VR3 = 4K7 multiturn (recommended) | 68-0420 | UH24B | 38-299 |
| single turn (cheap option) | 67-0525 | WR41U | 38-207 |
| Capacitors all electrolytic | |||
| C1 = 10uF 25V | 11-0220 | AT98G | 15-853 |
| C2 = 47uF 16V | 11-0325 | AU01B | 15-507 |
| C3 = not used | |||
| Semiconductors | |||
| IC1 = LM4558 dual op-amp | 82-0716 | QH46A | LM1458 |
| IC2 = LM3914 led bargraph | 87-0712 | WQ41U | LM3914 |
| IC4 = 4N25 opto-isolator | 58-0810 | AY44X | 4N25 |
| D1-D10 = LEDs | 72-8978 From Rapid are the ones I used but any 3mm leds of the colour(s) of your choice should do. | ||
| Array (RED) | 55-0190 | BY65V | n/a |
(Radioshack do the all red version code 276-081) and 25-499 for the red/green |
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| Misc | |||
| Pin Headers | 22-0565 | JW62S | 21-198 |
| Jumper links | 22-0692 | UL71N | 21-165 |