decoder
I have decoder IC 74138 and I found almost all decoder make selected output low (0) and I want it high (1) and I don't want to connect NOT gate to all of the outputs (it is illogical).
What should I do to make output high? Or is there any way to use this one without buying a new non-inverting decoder? If there is not what type of decoder should I buy (IC number)?
A simple circuit can be built using a 4017 counter.
The circuit normally counts 10 input pulses but by connecting the next output to the reset input via a small signal diode (eg. 1N4148) the 4th count resets the circuit to 0. It also allows you to expand the circuit very easily.to more outputs if required.
Edit (additional)
As OQ requires change when switch or signal input goes low then the clock input needs to be inverted. This can be easily accomplished with a single PNP transistor. A 100R resistor has been added in series with the switch to prevent it getting 'sticky' when discharging the capacitor (see Spehro's comment).
Also added the 10k between D1 and C2/R3 to the original circuit.
Maybe you could use a combination of 74HC138 decoder and 74HC245 bus buffer. Switch the 74HC245 off (into z state) for the all zeros state and put a pulldown resistor array to outputs.
Best Answer
It is natural for TTL to use active low signaling. This is because of its polarity asymmetrical nature. Low output drivers are typically stronger than high output drivers. Also unlike CMOS, TTL input switching voltages are much close to 0V than Vcc. Furthemore, you ca directly tie logical input to 0V, but not to Vcc, you need a resistor there. This cause open-low outputs are much more common than open-high ones (I cannot remember single chip with them). While it is better in many cases driven wire correspond to active state (e. g. for Wired-OR and hot plug) it is better to have low state as active.
So it is natural when designing TTL circuits to have active-low signalling.
If you still want design active high circuits you should look toward CMOS (4000 series chips): 4028, 4514, 4555 are decoders with active-high outputs. Also there 'migrated' chips like 74HC4028 which are (TTL-compatible) copies of corresponding CMOS circuits.