delayed constant to momentary relay

I would like to automate a push-to-make switch roughly 20-30 seconds after the car is started. The push to make can last anywhere from a 0.1 - 4 seconds. (It will activate the LPG system in a BMW). I have an ignition switched 12v and have used the following circuit for the momentary press. https://www.the12volt.com/relays/page5.asp#ctm I need to add a delay before the momentary press so tried adding another RC (C between switched 12V and gnd) to drive the circuit above but it's not doing what I need. I think there is a problem with the caps holding charge and interfering with the delay activation of the first circuit. But even if I discharge and give 12V the relay latches on without releasing. Any suggestions most welcomed. I was thinking about using ICs (monostable) but then I would eventually need to use a relay or optoswitch for the button press; so I was hoping it to be possible just one or two relays.

Yeah, the caps get way too big in those circuits, plus they can have oscillation problems. For 30 seconds a monostable (555 timer) is probably best, hence 2 555s. (I've preferred separate 555s rather than the dual version.) Or if you are game for something with infinite possibilities and instant circuit changes - which has negligible current drain; uses fewer components; and is cheaper - look at the PICAXE 08 (08M2). 555s should drive a relay direct (200mA) but otherwise use a transistor or MOSFET (pic 08M2 included).

Just found this on ebay. I'm thinking to use this for the delay part, then the above circuit for for the momentary switch. I don't need anything more complex and current drain is not an issue since it will be ignition switched. I was just hoping to do this the traditional way with a few RCs and relays.

oldspark wrote: Or if you are game for something with infinite possibilities and instant circuit changes - which has negligible current drain; uses fewer components; and is cheaper - look at the PICAXE 08 (08M2).

Wow this PICAXE stuff is very neat. I am a little out of touch with electronics and I can think of 100s of things I would do using this.