# 555 chip and continuity

When I place probes on pin 1 and pin 8 of a 555 timer chip, should there be continuity?

I'm trying to put a circuit together, actually a circuit to test if a 555 is good or bad, and can not

seem to get it to work.  Wondering if the chip I'm trying to use is already bad....

Thanks.

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That should sort of work.  The biggest problem with it is that the output (pin 3) won't go all the way up to the supply voltage.  It will be about 1.5 volts lower.  That voltage difference will be enough to light most leds.  Also, if the blink rate is too fast, it will "appear" they are both on even when they aren't.  With the values shown in the circuit (68K, 39k, and 1 uF) the blink rate should be about 3 or 4 times a second.  That's about the fastest where it is easy to see both blinking.  If you try taking the top led out of the circuit completely (the one connected to +9V  you should be able to see it working.  You can increase the size of either resistor or the capacitor in the timing circuit (68K, 39K, or 1 uF) up to about double and slow it down.  If you REALLY want both leds, put another led (or 2 to 3 regular diodes) in line with the top led to drop the voltage.

along with trying to get the motor controller circuit working (the PWM approach), I've been trying to build this breadboard

which will test the 555 chip.  When you hook a 9 volt to it the LEDs are supposed to alternate blinking (if the chip is good).  I've only been able to get both LEDs to light up at the same time.  Attached is a link to the circuit I've been working with.  If you could take a look at it.... does it seem like it should work?  I guess maybe the next thing is I should upload a picture of my breadboard... kind of frustrating.... but thanks again.  Learning alot with this exercise... but still, frustating.

http://i248.photobucket.com/albums/gg198/bocode/555-timer-test-circuit_zps55107e2a.jpg

That circuit should work fine but try swapping C1 for a 10uF or even 100uF. You say both LEDs are on at once which indicates it may be working fine but just going too fast for you to see the LEDs turn off.

Are you using a CMOS or TTL 555 timer?

The TTL timer and sink / source about 200mA max.
The Cmos timer can sink 100mA, but can olny source 10mA which may not be enough and make both LEDs to appear on at once.

A couple more notes:

As Oddbot said, that test is not a good idea.

The original motor control circuit might actually work, after all.  I have never seen that particular configuration before, but here http://www.dprg.org/tutorials/2005-11a/index.html is an explanation of how/why it works.  In my defense, the 555 is probably the most (ab)used chip ever, with lots of configurations never intended by it's designer.  I still recommend using a simpler circuit to start with, though.  It will be easier to get working.

As a teenager I constanly (ab)used the 555 timer. Not only did I use it as a timer, PWM and musical instrument but also as a latch and a comparitor! I turned that chip inside out!!

Pins 1 & 8 are the power pins. Trying to test if the chip is good by doing a continuity test on the power pins is pointless and it's possible you could damage the chip by doing this.

thanks very much for the replies, bdk6.  I spent most of yesterday afternoon and evening

trying to get these circuits to work and no love.  I keep ripping the things apart and starting over :-(

I'll try your suggestion of swapping pin 3 for 7.... and I'll take a look at the link you provided.

Thanks, I appreciate the help.

check the rest of the schematic.  I didn't look at it closely.  I immediately noticed that pins 3 and 7 were wrong.  There may be other errors.  That circuit is more complicated than you need for what you are trying to do.  The potentiometer with the two diodes is a way to keep the frequency constant while varying the width of pulses.  A  "normal" 555 astable as shown in the tutorial I linked changes frequency when you change the pulse width.  That shouldn't make any real difference in your application.  I haven't used a 555 in several years, so go with the tutorial.  It looks pretty good.

In your previous post about controlling the speed of a motor, the circuit you show is wired wrong, as is the one you got it from.  Pin 3 of the 555 is the output, not pin 7.  Here is a tutorial on the 555 http://pcbheaven.com/wikipages/555_Theory/

With that tutorial you can use the astable circuit with the same transistor output you used before to control the speed of the motor.  Remember the 555 requires a minimum of 5 volts to operate with most versions.

http://www.linuxfocus.org/English/July2002/article239.meta.shtml

About halfway down is a schematic of a 555.  The three resistors from pin 8 (Vcc) to pin 1(GND) total about 15 k OHM.  But that really won't tell you anything about whether the chip is good or bad, unless it is open circuit.