• on Computer/Software/Internet
  • Published in issue 1/2010 on page 32
About the article

USB Magic Eye

Valve indicates CPU load

USB Magic Eye

Herwe use a green glowing valve as a CPU activity meter. Power and control are both provided by the USB port. A simpler variant on the circuit, using a moving-coil meter, is also described. For the CPU meter we chose the USB stack described in application note AVR309 by Igor Cesko.

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Gerber file

CAM/CAD data for the PCB referred to in this article is available as a Gerber file. Elektor GREEN and GOLD members can exclusively download these files for free as part of their membership. Gerber files allow a PCB to be produced on an appropriate device available locally, or through an online PCB manufacturing service.

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Extra info, Update

- display of CPU load as a percentage (0 to 100)
- power and control over USB
- ATtiny microcontroller with software USB interface
- two variants with the same firmware
- display using moving-coil meter or EM84 ‘magic eye’ valve
- very low component count
- low-cost, readily-available indicator valve
- current consumption (moving-coil version): less than 100 mA
- current consumption (magic eye version): less than 500 mA
- freely downloadable PC software and microcontroller firmware, with source code

R1 = 1.5kOhm
R5,R6 = 15kOhm
R2 = 22kOhm
R4,R7 = 47kOhm
R3 = 330kOhm
C6, C7 = 22pF
C2 = 100nF
C1,C3,C4 = 100nF 250V
C2 = 100µF 25V
L1 = transformer; coil former # B66418WL008D1 with clip # B66418B2000 and core # EFD20 B66417GX187
D1 = LED, red, 20mA
D2,D3 = BY448
T1,T2 = IRLU014
T3 = MPSA42
IC1 = ATTiny2313-20PU, programmed, Elektor Shop # 090788-41
X1 = 12MHz quartz crystal
K1 = USB socket Type B
V1 = EM84 (6FG6) and 9-way (noval) valve socket
PCB, Elektor Shop # 090788-1
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