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enter image description here I've made a 555 cascading timer. First stage is approx 90 seconds, second is 30 seconds.

The first two led's light up in sequence as expected, but the 3rd doesn't light. I've checked all connections. I can't figure out why the 3rd light won't trigger.

Is there anything in the schematic that gives a clue?

Circuit is running on a 9V battery. The push button connecting to pin 4 of each 555 is just to reset the circuit

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  • \$\begingroup\$ if you're sorry about the schematic quality (I've seen worse), why won't you at least rotate the image so that I don't have to go 90°-necked to read it? \$\endgroup\$ – Marcus Müller Mar 9 at 22:47
  • \$\begingroup\$ Sorry Marcus every time I upload it it's flipped by 90deg \$\endgroup\$ – Chris Kelly Mar 9 at 22:57
  • \$\begingroup\$ that's not been my experience and it probably just means that your picture is actually flipped on your machine, but just displayed horizontally for some reason. \$\endgroup\$ – Marcus Müller Mar 9 at 22:59
  • \$\begingroup\$ I went ahead, did two clicks on imgops.com and added a rotated image. Cheers! \$\endgroup\$ – Marcus Müller Mar 9 at 23:11
  • \$\begingroup\$ By the way, this is a fine exercise in 555 design, but since three NE555 are more expensive and with all the analog components more error-prone, one would simply use the cheapest microcontroller one is willing to program for such a problem. Reduces your component count by about 17 capacitors and resistors. \$\endgroup\$ – Marcus Müller Mar 9 at 23:15
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You have 3 related problems. First, a standard NE555 cannot handle resistors in the megohm range very well. Second, the NE555 is a totem pole design, so while the output can get close to zero volts, it can only source Vcc - 1.2 volts. Third, the NE555 cannot source much current, so with an LED load the output is less than Vcc -1.2 volts. This leaves little headroom to trigger the next stage by a sudden drop in voltage, as the drop does not cover enough range to trigger the next stage.

Fix problems: First of all a CMOS TLC555 consumes much less current, with outputs that swing almost rail to rail. Second, the TLC555 can make use of resistors as high as 20 megohm. At that value the leakage of the timing capacitor becomes an issue. Third, whether you use the NE555 or the TLC555, it is best to drive LEDs with a NPN buffer, such as a 2N2222 or 2N3904. This literally takes the load off the timer. A viable option is to use 2.2K resistors in series with the LEDs. For the NE555 a 4.7K resistor from pin 3 to Vcc will help the output get to a higher voltage.

Right now I think your first 2 stages are working by accident-or divine province. First step could be trying a 2.2K resistor in series with the LEDs. If they all work as expected, you need do no more. Todays LEDs can be bright at 3 mA of current. Due to low output voltage and the need for many LEDs to have 2.9 volts minimum to turn them ON dim, run either the NE555 or TLC555 at 12 volts if possible.

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  • \$\begingroup\$ Thanks for such a detailed answer Sparky \$\endgroup\$ – Chris Kelly Mar 10 at 0:57
  • \$\begingroup\$ I was going to add a schematic of a single 555, but not sure how simple or complex to make it. I was hoping to give you enough clues to help out. \$\endgroup\$ – Sparky256 Mar 10 at 1:10
  • \$\begingroup\$ No need - your original answer was awesome. Thank you again. \$\endgroup\$ – Chris Kelly Mar 10 at 8:40
  • \$\begingroup\$ I've replaced all three 555's with CMOS versions, and now driving each LED with an NPN transistor instead of directly from the 555's. The final LED still fails to light in the sequence. The voltage at pin 2 of the last 555 is 8.74V at the start, so i'm still not sure what the issue is. I even tried running at 15V and still no joy. Could the problem be that the 22n cap on the second 555 is too large? BTW pins 6 and 7 of the last 555 are connected (schematic was wrong). \$\endgroup\$ – Chris Kelly Mar 13 at 23:43
  • \$\begingroup\$ The 22nF capacitor along with a 100K resistor at pin 2 create a long negative trigger pulse. This is too long of a trigger pulse. Keep the capacitor to 10nF if possible and change 100K to just 10K. The negative trigger pulse only needs to last 100 ns or so to work. Make sure pin 3 is close to zero volts when LED is OFF. Either the 22nF is leaky or the third 555 has gone bad. Easy to damage logic IC's. \$\endgroup\$ – Sparky256 Mar 14 at 0:41
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Connect pin 6 to pin 7 on 555(3) if it isn't already.

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  • \$\begingroup\$ Check that the 10nF cap on pin 5, 555(3) hasn't gone short circuit. The voltage on pin 5 should be about 2/3 of the supply voltage or about 6V with a 9V supply. \$\endgroup\$ – James Mar 16 at 16:54

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