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Been working on this for a while. Circuits are not my strong point. Power Source =5.5v dc The first image is a 4093 NAND that another used helped me with. It changes a dual led from one color to the other and sounds the buzzer for 750ms each time the 5v touch pad is pressed. The second image is the 5v relay and an NE555. This circuit will engage the coils relay and hold it until C1 discharges. What am I saying. You guys know this stuff. Let me get to the problem I'm having. Well it's more of a concern. I've connected pin2 from the output of the touch pad and pin4 from the output of the gate (where it connects to the buzzer) when the touch pads trigger goes high it causes pin4 to go high. At the same time the output of the gate goes low sounding the buzzer and pin2 also goes low which triggers and starts the 555. In theory it works but my concern is that the two pins may trigger at the same time causing unpredictable results. My question is can pin4 be delayed? Perhaps with a simple resistor? I've tried to figure this out but not having any luck. I do thank you in advance for your help NAND GATE 555 Relay Decade Counter

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  • \$\begingroup\$ this web simulator has many circuits in the menu tinyurl.com/y65tfnat. Learn from them \$\endgroup\$ Commented Apr 19, 2020 at 1:51
  • \$\begingroup\$ Awsome find. I'm not a circuit pro by any means but I'm fascinated by what ic's can do. \$\endgroup\$ Commented Apr 19, 2020 at 2:43

2 Answers 2

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Well technically you can use an RC circuit to create a delay to when pin 2 is triggered but a better answer (I believe) is this after a little information: For the 555 timer to work properly, in either astable or monostable mode, pin 4 must be connected to positive voltage (at least 0.7 V according to Datasheet) prior to any input trigger voltage. This is because this reset pin works to disable the 555 timer when it’s held low. So a problem could arise if pin 4 (reset) is made high and pin 2 is made low at same time. To solve this problem, you’ll want to attached pin 4 to positive supply, and just use the pin 2 trigger as you were. When the pin 2 is high, your relay will be off, and only when pin 2 is triggered low, will the relay be on for the brief period determined by RC circuit attached to 555 timer.

Also if you want to really want to give your piezo buzzer a 750ms on time, then you should consider removing the R1 and C1 connected to the nand gate. Instead connect the output of touch sensor to nand gate (connected as NOT gate like you have) then connect the output of that nand gate to pin 2 of a second 555 timer which is configured in monostable mode with RC network to give a 750ms period. Hope this helps.

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  • \$\begingroup\$ Leoman12 What great info. When the rest of my components get here I will put this to the test. I did not know that pin 2 operated that way. I thought it needed only a pulse not a toggle like you describe. I read that if pin2 is held low it would never time out. And even if given just s pulse and other pulses sent would be ignored. Only triggering pin3 can cancel (reset) the timer \$\endgroup\$ Commented Apr 19, 2020 at 2:33
  • \$\begingroup\$ Sorry my editing times out. I wanted to say that if pin2 receives any additional pulses while the 555 timer has started they would be ignored. \$\endgroup\$ Commented Apr 19, 2020 at 2:40
  • \$\begingroup\$ Sorry I need to clarify that when I meant trigger pin goes low I actually meant it is triggered low and then returns high. It shouldn’t be kept low. I was under the assumption that your sensor outputs a brief ON signal when touched. Is this the case or does your sensor hold a high output state when pressed and then holds a low state when pressed again? \$\endgroup\$
    – Leoman12
    Commented Apr 19, 2020 at 22:46
  • \$\begingroup\$ circuitbaker404, let me ask you , Is your touch sensor the TTP223 Capacitive Touch Switch? Also would you prefer your led, relay and piezo on when you press the touch sensor then off only when you press touch sensor again or would you be okay with the led , relay, and piezo on for brief time then off when you press the touch sensor once? \$\endgroup\$
    – Leoman12
    Commented Apr 19, 2020 at 23:38
  • \$\begingroup\$ To answer that I'd have to tell you the purpose of the led, speaker, and relay. The led and buzzer are visual and audible reactions for the user that presses the plate that is connected to the ttp223. The led must stay illuminated at all times only changing color from red to blue to visually let the user know they pressed the plate. And, each time the plate is pressed the buzzer sounds \$\endgroup\$ Commented Apr 20, 2020 at 23:17
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I’m just going to explain using another answer post.

Ok this clarifies things. So I’ll have to tell you that from my knowledge of the 555 timer and from the Datasheet they indicate proper use of 555 timer in monostable mode when the trigger pin is only triggered momentarily and not latched. Actually when it’s latched to a low voltage I’m not entirely sure what happens. It may latch the output and not work like desired.

So the circuit involving the LED, NAND gates, and the RC circuit with piezo would work as planned since you indicated that your sensor latched it’s output. However as stated above, I can’t guarantee the relay circuit to work even with my modification. I believe a better option here would be to use an arduino or some other microcontroller that would turn be able to turn on the relay and piezo buzzer for desired time when one of the pins is rising edge triggered. Your led circuit can remain though. And you would have to interface transistors with microcontroller to turn on both the buzzer and relay.

It seems there is a limitation of this 555 timer to your application.

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  • \$\begingroup\$ I added a new image. This circuit is pretty cool. It uses a 4017 counter to flip flop and start and stop the 555 ic. There touch plate is just a plate of aluminum. I would connect the output of the buzzer to this new circuit (removing their plate) \$\endgroup\$ Commented Apr 21, 2020 at 3:45
  • \$\begingroup\$ Yeah you could try that. You still need that RC circuit in your original diagram. You wouldn’t need the cd4017 since the 555 timer is already operating in monostable mode and can handle driving a BJT. Also since the negative side of buzzer gets pulled low, you should remove that Q2 since trigger pin of 555 timer would only active when pulled low (then returned high) \$\endgroup\$
    – Leoman12
    Commented Apr 21, 2020 at 14:12
  • \$\begingroup\$ Yes. Removing g2 would be just what I'd do since their trigger is different from mine. But the 4017? I thought it would solve my trigger and resetting of the 555 with the same button problem? But then again I do not know all that much about ics. \$\endgroup\$ Commented Apr 22, 2020 at 1:31
  • \$\begingroup\$ Well since the RC circuit connected to NAND will only allow negative terminal of piezo be pulled low temporarily it will pull trigger pin of the 555 timer low and then return high. No reset of 555 timer needed since in monostable mode, it only outputs brief pulse determined by R and C connected to 555 timer. \$\endgroup\$
    – Leoman12
    Commented Apr 22, 2020 at 2:04
  • \$\begingroup\$ Also you’ll need large values of R and C for 555 timer to get 10-15 minutes. Try 470Uf and 1M or higher. Or use larger capacitor to get smaller R. \$\endgroup\$
    – Leoman12
    Commented Apr 22, 2020 at 2:15

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