power on delay using a 555 chip

Might be easier to just use two 555s. One to trigger the other after 20 minutes and the triggered one to send the pulses. Both these circuits are in the app notes. Tom

There are whole sites devoted to novel ways to use the 555. Try google

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1. Why don't you do your own legwork?

1. 20 minutes is quite a long time for a 555. How accurate do you need the 20 minute timeout to be?

2. Once the pulses have started, how long do you want them to continue?

1. Please bottom post.

-- John Fields Professional Circuit Designer

--- I've rearranged your post to give you an example of what bottom posting looks like. Also, you may want to read:

Particularly:

"Summarize what you're following up.

When you click "Reply" under "show options" to follow up an existing article, Google Groups includes the full article in quotes, with the cursor at the top of the article. Tempting though it is to just start typing your message, please STOP and do two things first. Look at the quoted text and remove parts that are irrelevant. Then, go to the BOTTOM of the article and start typing there. Doing this makes it much easier for your readers to get through your post. They'll have a reminder of the relevant text before your comment, but won't have to re-read the entire article. And if your reply appears on a site before the original article does, they'll get the gist of what you're talking about."

If you bottom post I'll help you. If you don't, I won't.

-- John Fields Professional Circuit Designer

Accuracy within +10% to -10%.

Once the pulsing has started it should continue till mains is disconnected.

Hi, Kfel. From your earlier conversation, possibly this might fill the bill (view in fixed font or M$ Notepad):

| | VCC .--------------------. | + | | | 16| | VCC VCC | | VCC .--o---------------------. | + + | | + | Vdd | | | | | | |.01uF | | | .-. .--o--o--. | --- | | | | | | 8 4 | | --- | |3 | | | | RST | | | 12| CD4060 Q14o---o '-' | | | o-------oRST | | | | | To | | | | | o----o7 |Triac | .-. | | .-. | | 3o-->

| | |10K | | | | .-. | 555 | | | | | Vss | | |15K | | | | | '-' '--o----o-----o------o---' '-' | | .-o6 | | | 8| 11| 10| 9| | '-' | | | | === === .-. .-. | | | | | | | GND GND | | | |56K | V o--o-o2 | | | | | | | - | | 1 5 | | '-' '-' | 1N914| --- '--o--o--' | | | | | --- | | | .-. | | | === | | .->| |100K | | === GND | | | | | | | GND | | | '-' | | | | | | || | | | '-o---o--||--' | | | ||1uF | | '--------------' | (created by AACircuit v1.28.6 beta 04/19/05

Sorry you can't do this with a 555 alone, but 20 minutes is just too long for +/-10% accuracy. As an alternative, try the CMOS CD4060 along with your 555. The 4060 kind of revolutionized industrial time delay relays back when the earth's crust was cooling, and it can work fairly well for your application, too. Look at the data sheet for reference:

Internal to the 4060 is the makings of an internal oscillator, which can be set up either with a crystal or an R-C network. The components above are chosen so it will oscillate at between 3Hz and 9Hz, depending on the pot setting. You'll have to tweak it in, because the frequency of oscillation is somewhat dependent on power supply voltage (you do have a well-regulated DC power supply between +5V and +15V here, right?). Look at one of the intermediate outputs, and tweak it in with a frequency counter if you want exact results.

The thing is, to get +/-10% repeatability, you'll need a cap that can do that. Your repeatability is going to be almost entirely dependent on your cap. I'd look at mylar or one of the polys -- stay away from electrolytics and tantalums.

Anyway, after 20 minutes, the Q14 output will go high. That will forward bias the diode, pumping current into the summing junction and swamping out the oscillations. The 4060 will stay stuck until you cycle power, which is what you want. It also puts the RST input of your 555 high, allowing it to oscillate, and drive your triac as requested.

By the way, as a simple precaution, make sure the 1N914 is shielded from light if it's got a glass case. Light will mess up the diode leakage current.

Would you like fries with that?

Cheers Chris

Thanks a million chris. Will try it out.

Trust, but verify. The circuit should work at any 555 supply voltage (5 to 15VDC), but make sure it's regulated. Check the math for choice of Rs and C ( f = 1/(2.2 * R1 * C) ). The R2 (coming off pin 11) should be 330K or so. The diode and 15K resistor business is an example of the time-honored MML logic family (Mickey Mouse Logic) -- great for hobbyists, kind of something to be embarassed about if you're actually getting paid. It should work, though. You may want to consider using a spare gate or something if someone's looking over your shoulder, or inhibit the oscillator in another way.

Either that, or just brazen it out -- grin and say, "Well, it works, doesn't it? Haven't you ever heard of Wired-OR?" ;-)

Cheers Chris

Chris, I did try out the circuit but went about the testing in a phased manner. Step 1: I rigged up the 555 in astable mode. Provided it with supply from a 9V battery. Checked the output pin for oscillation and found it to be ok. Step 2: Then I rigged up a simple DC supply circuit using a 5.2V zener in parallel with a 470uF/16V capacitor. (This circuit is explained in the application note 954A of microchip corporation). Using this as the primary source of the power to the 555 i checked the output pin for oscillations and found it to be ok. Step 3: Rigged up a circuit containing of a 40W GLS lamp in series with triac BT06600 from ST Micro. Connected one end of the lamp to the MT2 terminal. Other end of the lamp was connected to Phase (240Volts AC RMS). MT1 terminal was connected to Neutral. Connected a wire from gate terminal but did not connect it to the output of the 555 as yet. Step 4: Switched on the power supply and checked the output at pin 3 of 555 and found it to be ok. As the gate terminal was not connected the lamp did not glow. Now when i connected the gate terminal to the output of the timer the 555 went kaboom. So did the triac. Unable to understand what went wrong. Any help would be appreciated.

If you wired the power supply like Microchip shows on their application note, you're lucky you're still alive. Notice that the output of their supply is connected directly to mains HOT, so with your TRIAC's MT1 connected to NEUTRAL, as soon as you connected the wire from the 555 to the gate you were connecting mains HOT (through the 555) to the gate, and then to NEUTRAL. Kaboom is right!

Hi, Kfel. Sorry for your troubles.

The consensus of this group over time has been that the transformerless power supplies shown in the Microchip appnote are not suitable for anyone who asks for advice from this group.

Mr. Fields was kind enough to mention why your 555 blew up. Your circuit common in Fig. 10, 11 and 12 are not at the same potential as neutral. For Fig. 12, it's a long ways away!

I'd like to add that there are any number of other ways to do this which don't involve putting your circuit at line voltage potential. First, your 555 can drive a small relay directly, or it can drive an NPN transistor which can switch the relay coil. If you're not interested in a relay, you can use an optotriac IC to drive your triac. If you want an authentic '70s feel to your circuit, you might have your 555 drive the reset pin of another 555, which drives a pulse transformer. The pulse transformer output can easily drive a sensitive or non-sensitive gate triac or SCR.

The common element to all of these is that you're using an isolated power source for fun and safety. A throwaway wall wart should me more than sufficient, if you provide a voltage regulator of any kind that can handle the current of your circuit.

If you'd like advice on any of these, feel free to ask.

Play safe, and have fun. Chris

Chris,

The interesting part is that when i connected the output of the power supply (using microchip's design) via a 3K9/2W resistor to the gate of the triac (bypassing the 555 altogether) it worked perfectly. Is it that i should have connected some resistance in series (say 820ohms) with the output from the 555 before applying it to the triac?

No, sorry, Kfel. I guess I wasn't being clear.

You *SHOULD NOT* use the transformerless power supply shown in the Microchip appnote. I'm not giving advice on what is a ultra-cheap, lazy and potentially hazardous way to power a circuit.

I'm not saying it's not done. But for a newbie (note your first question), this type of power is too dangerous. If you want to talk about any other option, go ahead. But I can't help you here.

If you want to talk about turning on line voltage stufff, get a wall wart first so you have an isolated power supply. But I'm not going to give advice on something that could potentially cause you injury. Believe me, nothing personal at all. Just safety first here. Sorry.

Good luck Chris