Initializing selection of radio button circuit on power-on.

Thread Starter


Joined Jan 7, 2021
I've been working on a radio button circuit originally posted by crutschow and thinking about how to initialize to a default selection at power-on since there's no initial selection indicated at the inputs A0-A2 of the 4514.

1613494588792.png I'm using this to drive some relays for a remote antenna ham radio project (K9AY loop selection) and have a couple of NAND Gates left over so I was thinking to present an input "high" on A0 of the 4514 using an RC charging circuit.

Here's a portion of the KiCad schematic of my circuit with the added NAND gates (I added these later, thus not the nicest layout):
1613495643212.png Outputs of the 4532 encoder O0 - O2 feed the inputs A0-A2 of the decoder (I'm using the negative output 4515 instead of the 4514). At power on, the capacitor should begin to charge and hopefully hold the input long enough to keep input to A0 high long enough to default selection of Q0 output to low so switch 1 is selected at start up.

Will this work?



Joined Mar 14, 2008
To set the 4515 outputs at startup I believe you also need to pull EL (pin1) momentarily low, which enables the input latches.

It should be easier just to momentarily pull input I1 high on the 4532 during startup.