metering selector

elektor july/august 1985 before make. Unfortunately, tran- sistors have the property of switching on much faster than switching off. For example, a well-driven BC 547 takes a couple of 1.4s to switch off, but far less than that to switch on. The present circuit circumnavigates these potential troubles by using the output level as criterion, whereby a 4028 serves as the referee. The 4028 is a one-of-ten active high decoder which drives one of three transistors, T1. . .T3. Let us assume that Ti is on: its collector volt-ge is low, and so is input A of the 4028. The other two collectors are hign, and so are inputs B and C of the decoder. The 4028 therefore sees binary code 6 (110) at its input and this causes pin 6 to go logic high, so that T1is driven hard. When in this condition another key, for instance, S2, is pressed, a wrong code, i.e., 4 (1001, ensues. Output 4 of the 4028 is, however, not connected, T1switches off, but T2 is not yet driven. Only after T1has actually EMI A 14....