For some reason or other, the Uk.0 principal feature of a PC is reckoned by many to be its CPU clock speed. and that is probably why many PC users have developed a habit of judging a ‘C by what appears on the 2-digit speed readout. Machines with a display reading -low ’33’ (for 33 MHz), or which lack a display altogether 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-Mliz AT with an I’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 gray desktop case. Joking apart. resistors R1-R14 limit the LED segment currents. and their value TI”Tay be changed as required (within limits. of course) to reduce or increase the read-°’:1 brightness. The ‘turbo’ speed indicator-lion is switched on via transistor T2. Applying +5 V to the base of ‘F2 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 x3 into x2 (2) in the fifth (Turbo’) column, change all 5s into 6s.
