[bluesman123]

I've tried to figure out how those relays are wired that set off the buzzer when you turn off the engine an things like lights are still on. How do you trigger the relay??

Thanks for your help,

Dave

[docvette]

Doc here,

A relay is a two part controlled switch... It consists of a low current control circuit (coil), and a high current set of contacts .. the CW, NO, and NC terminals.

To operate one, say for a fan circuit, you need the Contact Terminals CW and NO..The NO will go to a fuse link direct to the battery, the CW will go to the Fan motor..these contacts handle the high current to drive the fan.

To control the relay you need the coil circuit, One side of the coil to ground, and the other through a switch or sender to 12 volts..

When the coil senses 12 volts and ground at the relay, it energizes a magnet that pulls the movable contact CW (or center wiper) Toward the NO (normally open fan circuit) and completes the fan circuit..at the same time if you have a set of NC (normally Closed circuit) on the set of contacts it will open that circuit as it energizes the fan circuit..

In this manner you safely can control a high current item..say 30 amps with a 1 amp source..AND deliver more current to the load.

Relays can be used for switching, reversing of motors, stepping circuits (start, low speed then run high speed) they can be used for redirecting circuits and just about any application that can be achieved with a switch.

Doc

[johnnymopar]

doc wasn't much help explaining how a relay works.

this situation is different because you have a logic problem, ie. when the ignition is OFF and the lights are ON you want a buzzer to turn on.

you will have to control a relay, possibly a timing relay, say 10 secs.

one way you might be able to do it is if you had an off-delay relay operate when you turn off your engine. you turn off your ignition and the off-delay relay would switch state and close a contact. with this closed contact you run a circuit through your lighting circuit switch to your buzzer relay. if your lights aren't on then the control circuit for the buzzer won't be a complete circuit and won't turn on the buzzer.

i know this sounds confusing - and it is. but i think this will work.

hope this helps.

JB

[docvette]

Doc here,

Sorry if I was less than clear....

TDR or Not..The relay NOMENCLATURE is no different than I previously outlined...except stripped to it's basics for presentation...

A TDR , While mechanically no different from any other...uses a R/C circuit or a linear Op amp to control the amount of delay preset for the function it is desired to control..R+C=t...where T= time R is resistance And C is capacitance... Combinations or different resistance and capacitance values will give you different time values...

For ON Delay, a control voltage is sent to the coil and R/C network..The capacitor then begins to charge from 0 to hero with a small leak rate to the coil...Different size caps Charge up at different rates, AND you can control that rate with a resistance to ground. By varying the amount of resistance also can vary the T factor...(For Example ONLY) 1000 MFD Electrolytic Cap and a 10 K resistor = 10 s, OR to Change, (For Example ONLY) 500 MFD electrolytic cap and a 5 K ohm Resistor = 5 s (No math was actually Harmed while writing this.. )

For "OFF" delay, the reverse is true... The different value cap and resistor combination (less leak rate to ground) Holds the coil high (closed) after the power Signal is removed Until the Cap reaches a point of discharge where it can no longer hold the coil closed..Different value Caps, Different Value resistors will result in a different rate of discharge, hence, Time value..

A Semiconductor type of TDR uses an op amp similar to a 555 timer and Still uses an R/C network, EXCEPT for instead of the cap and resistor being the sole values for setting timing values, are used as TRIGGER and THRESHOLD inputs to the timer Chip..By using these as a network, and adding a Variable resistor ( like a radio volume control pot) you have a range between X and X rates of charge / Discharge, By varying the NULL of the chip with the variable resistor, you can "Fine tune" a value between the full range of delay..instead of just one setting..(For EXAMPLE only)..0 to 9 on the variable resistor= 1 or 10 s + 1 S= 11 s..etc..all the way to 19 S...without having to physically changing a resistor or cap to add or subtract time values..

The timing network (no matter what type used) is placed before the coil of the relay..and fires the coil dependent on the configuration of the timing network..(On delay/off Delay) which in turn energizes (or de energizes) the magnet inside the relay at the end of the timing cycle, ( for our purpose lets say on Delay) pulls the movable contact towards the magnet (Cw or Center wiper) and either / or / or both..opens the NC (normally Closed circuit ) and Cw while at the same time Closes the NO (normally open circuit) and the Cw..

NONE of this will you ever be concerned with, (except for the timing value itself, unless you plan on build the r/c net yourself..) as the relay will look and hook up the same with one exception, the coil circuit is usually given polarity either via Diode strapped across the power and load, to keep the cap or chip from blowing up or shorting out on reverse polarity at the coil. (the coil pins will be + and -, where on a standard relay it dosen't care..) The Terminals will remain the same as a standard relay..NO, Cw, NC...and some will use a second set of external terminals as a "Latch" circuit to hold the contacts closed / open during timing cycles..

But the relay will hook up the same as a standard relay with the possible exception of polarity, and Time delay value , chosen by you when you purchase the relay..

To sound a Buzzer, Johnnymopars description will work fine, but you still can play with that by moving contacts around or combining a TDR and relay depending what configuration and what you want the circuit to do..Also most modern Buzzer's have a timing network built into them itself..so that is another way to go...

Everything you never wanted to know about TDR's...

Doc

[guba_unus]

Have you checked the knowledge board? This site has a wiring diagram showing how to setup exactly what your doing. This is similar to what JohnnyMopar said only with a picture

[bluesman123]

Thanks very much for all your help. Especially for the diagram reference. I did check the board but totally missed this link.

Dave

[guba_unus]

yeah, actually I was mis-leading, cause the link directly to that wiring diagram isn't on the knowledge board. But there is a link to another part of that same page, don't know if it is worth adding that one to the knowledge base, but don't feel dumb that you didn't find it.