• 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 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. Elektor recommends its business partner Eurocircuits as the company of choice for its own prototypes and volume production in selected cases.

Extra info, Update

Features
- 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

Resistors
R1 = 1.5kOhm
R5,R6 = 15kOhm
R2 = 22kOhm
R4,R7 = 47kOhm
R3 = 330kOhm
Capacitors
C6, C7 = 22pF
C2 = 100nF
C1,C3,C4 = 100nF 250V
C2 = 100µF 25V
Inductor
L1 = transformer; coil former # B66418WL008D1 with clip # B66418B2000 and core # EFD20 B66417GX187
Semiconductors
D1 = LED, red, 20mA
D2,D3 = BY448
T1,T2 = IRLU014
T3 = MPSA42
IC1 = ATTiny2313-20PU, programmed, Elektor Shop # 090788-41
Miscellaneous
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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