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Timer Circuit

I found this (http://jamesyawn.com/timers/index.html) simple circiut onjamesyawn.com, and was wondering if and how to change it so that it could time inthe minutes, longer than it does already. I don't know anything about electronics,aside from soldering, and basic connections, so try to give a simple-ish answer. I alsowondered about what transistor to use (like a radioshack part number). The circuit Iwas talking about is under "discrete timers".

(update, more questions)What kind of timing (like how many seconds) would I get with this?Using the same stuff shown on the schematic?

Also, how do I pick a transistor?Are they interchangeable? What specs should I be looking for?

If this seems like too much, could someone give me a link to a site that would bewilling to help with (very) beginning electronics?

AND, how would you recommend making the transfer schematics for home-etchedcircuit boards? I can't get the component spacing right.

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guitardenver (/member/guitardenver/)

555 timer is the next step, look for it on the internet. Radioshack sells it. Its simpleto use and I got up to 30min with it. But its not exact its more in between 25-35min.(it changes). You want the astable, or osillator mode when you research it, that willmake more sense when you look for it.

Austringer (/member/Austringer/)

A while back somebody offered me a heap of ego with very precisely zeroadvice on one of the Make comment threads. I'm going to give you the advisehe would have given me had he not been all about letting me know how smarthe was. Track down a copy of "The Art of Electronics" by Horowitz and Hill!It's a text book and, ergo, not exactly cheap, but I'm finding it worth it. Toanswer one of your questions up there, a capacitor charges in roughly 5RCwhere R is the resistance in ohms and C is the capacitance in Farads. In thediagram you have your looking at 150,000 ohms x .0002 Farads so 2. 5minutes to FULLY charge the capacitor. Your transistor will do it's thing beforethat happens. What are you wanting to do with this? How exact does it needto be?

gmoon (/member/gmoon/)

This uses a PNP transistor. Radio Shack sells the 2N3904, a PNP--but it canonly handle 100mA (I've seen small relays that only draw 30-40mA, but anautomotive relay is anywhere between 80-140mA....)

I'd use a 2N2222--cheap, easy to find, and can sink 1A. But they are NPNtransistors, so change the circuit: Transistor circuits(http://www.kpsec.freeuk.com/trancirc.htm) link.

You might consider using a 555 instead of the simple RC circuit....

Pretty old project there... Ironically, it's described as " Time delay relays arebasic electronic circuits which serve to protect costly equip. from damage dueto current surges ", but there's no surge-protection diode on the relay! Checkout the link above for an explanation of the diode placement.

John Smith (/member/John+Smith/) (author) gmoon

Ok, I'm lost. I need to find asimple electronics book.

Anyway, would a 2N3055 work?It is NPN, and I have one in thegarage.The specs of it are here.(http://www.kpsec.freeuk.com/components/tran.htm)(at the end of the NPN section)

Thanks for the help.

gmoon (/member/gmoon/) John Smith

I don't see why not...but that'sserious overkill for a relay! (15amp rating !!) It's gain (Hfe) israther small, so it will draw morebase current (no biggie.)

Most of the example on that link(http://www.kpsec.freeuk.com/trancirc.htm)are NPN, so start there. As faras converting your original

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Posted:Jun 12, 200724,242 views

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schematic (PNP), it's essentiallyupside-down compared to thePNP circuit on the 'TransistorCircuits' page.

I.E., (for your schem) the powersource polarity-- the GND (-) isat the top, and the positive is atthe bottom...Use the NPNcircuits, and convert the 'timer'part by flipping it top-to-bottom...

Both Goodhart and I made thepoint about spike protection, sopay attention...;-)

John Smith (/member/John+Smith/) (author) gmoon

GAH! I'm all confused!

...As far as converting youroriginal schematic (PNP), it'sessentially upside-downcompared to the PNP circuit onthe 'Transistor Circuits' page.What do you mean "upsidedown"? To what PNP circuit onthat page? I didn't see a timerthere.

I.E., (for your schem) the powersource polarity-- the GND (-) isat the top, and the positive is atthe bottom...Use the NPNcircuits, and convert the 'timer'part by flipping it top-to-bottom...What NPN circuit?

Both Goodhart and I made thepoint about spike protection, sopay attention...;-)*sigh* I guess now I have to askwhat the heck you are talkingabout...SO, what is "spike protection"?

I am SO sorry that I am makingyou do all of the work, I justdon't understand this stuff. I'vetried, but the second I get tounderstanding it, I get confusedagain!

gmoon (/member/gmoon/) John Smith

The "that page" link(http://www.kpsec.freeuk.com/trancirc.htm)reference...

1) What do you mean "upsidedown"? To what PNP circuit onthat page? I didn't see a timerthere.

--There is only one PNP circuiton "that page." Since you wantto use an NPN instead, you'llneed to convert your PNPcircuit to the way they drawthem on "that page..." Thepower source polarity on theirdrawing is upside-downcompared to yours....

--Yep, no timer there, that's why

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6 years ago Reply (CK2DEILFA0CVE79)

you want to convert yourschematic.

2) What NPN circuit?

--Any NPN circuit on "thatpage"--might as well get aworking transistor-switchedrelay before tackling the timer...

3) SO, what is "spikeprotection"?

See the Protection diodesection on "that page" :

If the load is a motor, relay orsolenoid (or any other devicewith a coil) a diode must beconnected across the load toprotect the transistor (and chip)from damage when the load isswitched off.....

There's a nice illustration nextto the text....

Forget it, I drew you a pictureand converted the circuit toNPN (No guarantees this willfunction, as I'm just copyingsomeone else's circuit...) Theright side is "their" drawing, theleft is the timer circuit fromyours...

(http://cdn.instructables.com/FMA/ENYZ/FA0CVDZR/FMAENYZFA0CVDZR.LARGE.jpg)

John Smith (/member/John+Smith/) (author) gmoon

Ok, thanks for clearing that upfor me. I just get confused withelectronics and stuff. Oh, andthanks for the schematic, too.

gmoon (/member/gmoon/) John Smith

Yep. Stick with it, it'll make senseeventually.

A couple things I didn't mention:

--The relay would rated for thesupply voltage ( 5V, 9V ,12...whatever.)

--The 200uF cap is polarized (hasa + and - side), so don't connect itbackwards. The top in thedrawing is connected to +V, sothat's the + lead...

--There isn't a base resistor, persay, just the RC timer circuitconnected to the base (maybeyou should add a current limitingresistor to the base.) But also, theamount of base current requiredto switch on the transistor will

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vary according to the load (therelay.)

So it's pretty much impossible topredict what the delay time willbe. Switch the load to an LED ormotor, etc, and the delay willchange...

John Smith (/member/John+Smith/) (author) gmoon

Ok, I think I got it, I'll solder oneup tomarrow...man, I wish I hadmy breadboard right not...I guessI'll have to wait for Christmas.

John Smith (/member/John+Smith/) (author) John Smith

Hey, that's a NC relay, right?

gmoon (/member/gmoon/) John Smith

Don't think it matters--choosewhichever type suits yourpurpose. Also, should havementioned--the relay voltagesource doesn't need to matchthe voltage applied to the timer /transistor base. In that case, the'initiate' switch wouldn't beconnected to the relay voltagesource. The switch could just beconnected to the timer part. Bothpower supplies need to sharethe same GND, of course... Forthis circuit, it doesn't matter. Butif you wanted to switch a 12Vrelay with a 5V microcontroller(instead of the RC timer) itwould...

John Smith (/member/John+Smith/) (author) gmoon

Any idea on how to design thetraces for etching my own board?I can't seem to get the spacingright.

gmoon (/member/gmoon/) John Smith

At this stage (if you don't have asolderless breadboard), I'd useone of those perfboards with thecopper pads. Be easier tochange components, since youhaven't tested the timer, etc.Just solder point-to-point withsome wire...

John Smith (/member/John+Smith/) (author) gmoon

I did that, and tested itonce..and...it WORKS! Well, Ihave a few kinks to work out, butI think it WORKS! I can hear therelay, but, it doesn't seem to stayon, just a short burst of clicks tobegin with, then silence, evenwhen I hold the battery on. I thinkit's because of a low battery; my9v I'm using is giving out 8.35v(even thought that sounds aboutright). I'll try it with two 6v lantern

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batteries later. You don't thinkthat'll be too much current, doyou? Gah- I know I'm getting abreadboard for Christmas,because I saw the box from TheElectronic Goldmine today!Dangit, I need it NOW.

gmoon (/member/gmoon/) John Smith

Yeah, could be a weak battery(especially if it reads low withouta load.) You need to choose thesupply voltage based on therating of the device. Running a6V relay on 12V forces morecurrent through than the relaycan handle. Your transistor won'thave a problem with 12V (max60V.)

John Smith (/member/John+Smith/) (author) gmoon

Don't think it matters--choosewhichever type suits yourpurpose.Ah, yep, stupid question.

For this circuit, it doesn't matter.But if you wanted to switch a 12Vrelay with a 5V microcontroller(instead of the RC timer) itwould...Ok, I understand that. Hopefully Ican find a working relaysomewhere, and make the circuittomorrow...

NachoMahma (/member/NachoMahma/)

. If you're talking about "A. Schematic from Radio Shack 150 Project kit (circa1972)", then it's just a simple RC (resistance/capacitance) circuit. . The timecan be changed by varying the resistors and the capacitor. Don't rememberthe details, but someone that knows will probably chime in. . It looks to methat just about any ?NPN? transistor that will handle the load of the relay coilshould work - this is not a "precision" circuit.

John Smith (/member/John+Smith/) (author) NachoMahma

Like I said, I don't know muchabout electronics.Would a "mpf 102" transistorwork?Also, could I use a variablecapacitor instead of the variableresistor? I don't have a variableresistor around, so...Also, when I put in the transistor,which way does it go? Accordingto this(http://jamesyawn.com/timers/150kit-schematic-776.jpg) page (flatside to the left/right)?

Goodhart (/member/Goodhart/) John Smith

The MPF102 is a MOSFETliinke to mpf102 specs(http://www.futurlec.com/Transistors/MPF102.shtml)

Goodhart (/member/Goodhart/)

http://jamesyawn.com/timers/index.html

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Does appear to be a broken link.

John Smith (/member/John+Smith/) (author) Goodhart

Oh, yep, sure is. his entirewebsite seems to begone...awww, I liked it... Theschematic I was talking about isabove, and there isn't much infoon that specific timer on that pageanyway.

Goodhart (/member/Goodhart/) John Smith

The only thing I see that mightbe a problem is the lack of adiode across the coil of therelay: potential for the creationof a backwards surge when thecoil opens each time. Thiswould surely destroy thetransistor.

John Smith (/member/John+Smith/) (author)

*ahem* BUMP *ahem*

NachoMahma (/member/NachoMahma/)

> Like I said, I don't know much about electronics. . It's been several decadessince I worked with this stuff so I can't give you any details. :) . > Would a"mpf 102" transistor work? . Don't know. I no longer have the references tolook it up. . > Also, could I use a variable capacitor instead of the variableresistor? I don't have a variable resistor around, so... . No. That one I do knowthe answer to. . > Also, when I put in the transistor, which way does it go?According to this page (flat side to the left/right)? . I don't remember. . . Sinceyou're not getting an answer here, you might try a site devoted to beginningelectronics.

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