Okay, I've got an interesting project I'd like to try, and I'm looking for tips.

I've got my radar detector stealth-mounted in my car now. I completely dismantled it and installed its guts in the console just above my rearview mirror. It works well, I've lived with it for a couple of weeks and it's already saved my ass a few times.

However, it has a feature I dislike, and there doesn't appear to be a way to disable the feature. The feature is that its power-on-self-test is very noisy. It plays one beep of each of all of its various warnings when you first turn it on. Thats about 5-10 seconds of really irritating beeping and buzzing each time I start my car.

I've e-mailed their customer service department to see if there's perhaps a secret handshake that will let me disable the noisy POST sequence. But failing that, I'm wondering if I can put a time-delayed relay on its speaker line. Since I control the power supply going into it, I'm wondering if there's an easy way to do this.

Here's what I envision:



Can this be done easily and inexpensively? Anyone have any ideas how to pull this off?

By the way, it occurs to me that this would be the same circuit that's used to time-delay the turn-on of an amplifier to prevent a pop. I could have sworn the schematic for such a circuit was once posted ont this BBS, but I can't find it in a search.

Dunno if this would work for ya but here it is. Delayed turn on

I'm not sure if that's the sort of thing I'm looking for, as it seems to go onto the amp remote control wire as opposed to an audio signal wire.

I would imagine it would still work if you put it on the postive side of the speaker cable

I would imagine it would still work if you put it on the postive side of the speaker cable

I don't think so because it says that it has only three connection wires: Input, output, and ground. This sounds like I would be putting 12v DC power directly into the speaker if I did it that way.

Anyone have any other ideas?

Okay, does this look right?

Are those kinds of parts available at your average Rat Shack?

I did a quick google search for a de-thump circuit (since that's really what you are looking for here) and found this really simple circuit. Sure it needs adapting to your needs but relatively speaking it's much less complex than the 555 timer curcuit.

I could have sworn the schematic for such a circuit was once posted ont this BBS, but I can't find it in a search.

Possibly John (who owns the site) posted something on this BBS, if you scroll down on that previous link you will see he's an empeg owner, it's a small world

Edit: <teaching grandmother how to suck eggs> This thread is perhaps the one you were thinking of?.</teaching grandmother how to suck eggs>

Well, it's too late, I already went out and bought the parts for the other version, the one I linked first.

Now I have only one question:

Which bloody pin is number 1 on this bloody chip???

Pin 1 is the one on the top LHS corner (nearest the indented circle).

Edit: Don't forget to do your Ohm's law calculation on the 555's max. output current (200mA) vs. the coil resistance of your chosen relay

I would have used an off the shelf $20 module such as this http://www.altronix.com/p_html/6062.htm

Pin 1 is the one on the top LHS corner (nearest the indented circle).

But in the photo, the circle is on the bottom LHS. So does the circle indicate pin 1 or pin 8?

Don't forget to do your Ohm's law calculation on the 555's max. output current (200mA) vs. the coil resistance of your chosen relay

The relay I have chosen is part number 275-0241. Its specifications are here. Will that work?

Note that my supply voltage for everything is 12vdc.

The dot indicates pin 1. The notch in the chip should be at the top when counting the pins.

[edit] Ignore the bit about the notch since your chip doesn't have one... Put the dot at the top left anyway [/edit]

- Trevor

Thanks!

Okay, watch out! Tony's got a soldering iron and he's not afraid to use it!

Your relay should be fine. Allowing for the diode (0.5v) and assuming your 12v might even rise to 14v the most you will be asking of the 555 will be ~43mA DC ( (14v-0.5v)/324R ).

Yes (as tman said), you should orientate the IC so that the indent is uppermost and Pin 1 will be top LHS. Sometimes you just have a notch instead of the circular indent and again, with that uppermost, Pin 1 will be top LHS.

It's kindof obvious when you know and less so when you only have one IC to play with

Okay, well maybe someone can help me with this.

The circuit was designed, as far as I can tell, with a temporary contact switch (S1) to trigger the timer.

I don't want a switch to trigger the timer, I want the timer to start as soon as power is applied to the system. So I figured just wire it with S1 as just a straight connection instead of a switch.

Well, that doesn't work. With very low values at R1 (anything close to zero resistance), the relay goes ON OFF ON in the space of about a second. With any higher values at R1, the relay just goes ON and stays there.

Can anyone examine the schematic here and tell me how to re-do it so that I don't need that temporary switch at S1?

I want it to behave as follows:

- Power is applied to the system.
- There is a 5-10 second pause.
- The relay clicks over and stays "clicked" from then on as long as power continues to be applied to the system.

HELP!

Okay, I'm trying to read the excellent 555 tutorial here. It's got all the information that I need to understand this, but oh boy it's tough to wade through. I don't speak this language normally, so it's hard to wrap my mind around it. Can someone please help?

The problem is that the 555 isn't really a single chip solution to this problem. It is designed to be triggered, take an output high for a preset period, then take it low again until the next trigger pulse. What you want is a circuit with inverted logic, ie trigger, delay, then output, not trigger, output, delay. Another problem is that the trigger input is specifically designed to be a momentary input, not a continously grounded one. You get odd behaviour if you take it low and keep it that way, and the behaviour changes from manufacturer to manufacturer.

You could use a 556 dual timer or two 555s to approximate what you want, but to be honest I've always found them to be a pain at best. The design is ancient, and all too analog for my taste. I would do it by programming up a PIC12C508 to give exactly the correct behaviour, but then I'm like that!

An analoggy way to do this is to use a suitable RC circuit to trigger a transistor or mosfet. The R and C are chosen to give a time constant such that the gate or base voltage goes above the switch-on threshold after the appropriate delay. The device will then stay switched on until the power is removed and the capacitor is discharged. This is pretty fast for a transistor, could be days for a mosfet without a resistor to help. Crude, but reasonably effective. A PIC is still easier, though.

I'm just off to bed, but I could sketch a suitable circuit tomorrow if it's any help.

Patrick

I only understood about 20 percent of what you just said. That's how dangerous I am with a soldering iron.

I was hoping there would be a way to just make this thing work with the stuff I bought today. I spent 15 bucks on parts, and that's about the limit of what I'm willing to spend on this project. If anyone can figure out how to do it on a low budget, you can.

Anyway, any help at all would be appreciated, and picture illustrations of course work well for me.

Oh, one thing I did want to say:

It is designed to be triggered, take an output high for a preset period, then take it low again until the next trigger pulse.

That's fine. The relay I bought is spdt. So I can deliberately wire it "backwards" so that it's normally closed and then it goes open when the chip's output goes high. I was figuring that the output could go high for ten seconds, then go low again and not ever get triggered again.

I just need to know how to do it without that momentary contact switch.

Isn't there a way to do this with just a capacitor, a resistor, and the double-pole relay?

Try this one:
http://www.andy-clarkson.me.uk/555/mono4.html

Replace the switch with a wire, use the output stage of the original circuit, and use the normally-closed contacts..As soon as the circuit powers up, the relay will switch on, breaking the speaker circuit. After t1 seconds, the relay will switch off...etc.

Ooo, thanks. Trying it now.

I'm probably to late, two new posts while I was drawing a schematic.

The attached is similar to what Patrick was suggesting.

Rod.

Oops, attached the wrong file. You probably can't open the last one.

Try this one.



Rod.

VERY nice. Thanks for the diagram! However, the circuit that Genixia suggested happened to use the existing parts that I already had sitting in front of me.

Genixia, initial tests indicate that your suggested circuit works and is exactly what I needed.

Pardon me while I dance a short victory jig around the room.

THANK YOU!!!!

This BBS is the best. I love all you guys. *sniff*

Oh boy, Genixia, that circuit really works well. Thanks again!

/me drinks wine cooler in celebration.

Now the hard part. Cleaning up this messy prototype, and making it neat and integrated.



And I have to turn that variable resistor into a fixed value while I'm at it.

Personally, I found that purchasing a breadboard made this type of work much easier.

Now if I only had the knowledge and/or skill to go along with it.