Low Voltage Circuit

Speed indicator for PCs

For some reason or other, the principal feature of a PC is reckoned by many to be its CPU clock speed, and that is probably why many pc developed a habit of judging a PC by what appears on the 2-digit speed readout. Machines with a display reading below ’33’ (for 33 MHz), or which lack a display all together, are not worth talking about. In other words, if your PC is not fast enough. you are ‘out’ before you know it. To fool these sufferers of ‘PC speed phobia’. and to bring you up to par again, we propose a circuit that will feign a much higher CPU speed than actually used in the machine. In this way, you will be able to accredit. say, an ‘old’ 12-MHz AT with a ‘state-of-the-art CPU turbo speed of 66 MHz, or even 99 MHz! After all, only you will know for sure what’s really ticking inside the grey desktop case.

Joking apart, resistors R1–R14 limit the -LED segment currents, and their value may be changed as required (within limits, of course) to reduce or increase the read-out brightness. The ‘turbo’ speed indication is switched on via transistor T2. Applying +5 V to the base of T2 causes T1 to be switched off. The reverse situation, T2 off and T1 on, applies when the turbo switch is set to the ‘normal’ position.

Unfortunately there exist differences between PCs as regards the logic levels used to switch between ‘normal’ and ‘turbo’ CPU speed. In most cases, turbo speed is selected when the switch contact is a ‘make’ type that closes to ground. The table shown applies to that configuration. The table is also valid if the ‘normal’ position of the switch makes contact to +5 V; only R17 must then be removed. In case turbo speed is selected when the switch contact closes to +5 V, the table needs to be modified, and R17 omitted. These modifications are also required, but R17 must be left in place if ‘normal’ speed is selected with the switch contacts closed to ground. The modifications to the table are as follows: (1) in the fourth (‘Normal’) column, change all entries *3 into *2

(2) in the fifth turbo column, changes all 5s into 6s.

Speed indicator for PCs 321
Speed indicator for PCs

Speed indicator for PCs Construction of the readout is simple on the printed circuit board shown (not available ready made). The board is cut into two to separate the jumper section from the display section. The two sections are interconnected by a 15-way SIL socket and a 15-way SIL header. Finally, note the wire ink that runs between the speed setting jumpers – if you forget to fit it. the displays will remain dark forever in the absence of heir supply voltage.

Speed indicator for PCSX
Speed indicator for PCs

Speed indicator for PC Parts list…


  • 14 270Ohme       R1-R14
  • 3 3kohme9           R15; R16: R17


  • 2 BC337                 T1; T2
  • 2 HD11050            LD1; LD2


  • 14   2-way pin header
  • 14   3-way pin header
  • 1      15-way SIL socket
  • 1       15-way SIL header
  • 1       Changeover switch  (turbo switch in PC)     S1
Speed indicator 55
Speed indicator for PCs

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