CD4027 Push ON/OFF Latch Circuit to Drive 5V Relay Using 3V Input Voltage

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How can I build a push-on/push-off latch with a CD4027 that runs from 3 V and switches a 5 V relay?

Run the CD4027 from the 3 V logic supply, but drive the 5 V relay through a transistor stage powered from a separate 5 V source or regulator; do not try to feed the relay directly from the 4027 output [#21660567] If you want a clean push-button clock input, condition the switch with an RC network and a Schmitt-trigger inverter such as a 74AC14, or use a double-pole, single-throw pushbutton into the S/R inputs; the Schmitt stage gives a sharp edge and prevents the 4027 from sitting in its active region during switch bounce [#21660570] Add local decoupling capacitors across VDD/VSS on the ICs and make sure unused inputs are tied high or low [#21660574][#21660588] If relay current is causing supply dips, route the relay current on separate supply lines so it does not share the same feed as the logic [#21660584] For the output transistor, PN2222A was suggested as a suitable choice [#21660586]

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#1 21660566 03 Jan 2012 22:26

RAJU MUKTAWAR RAJU MUKTAWAR

Anonymous

Post #1
21660566 03 Jan 2012 22:26

Hi, I want to build a PUSH to ON/OFF circuit using CD4027. I have two voltages. 3v & 8.5v
I want the circuit to work on 3v and drive a 5v relay at output.
Can anyone help me for this circuit?
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#2 21660567 04 Jan 2012 01:41

Steve Lawson Steve Lawson

Anonymous

Post #2
21660567 04 Jan 2012 01:41

Questions:
# You "have" 8.5v? Yet you want it to work with 3v? So, what is the role of the 8.5 volts? Why even mention that?
# Will your 5v relay work at 3 volts? How much current is needed to drive the relay at 3v or 5v? OR what is the relay coil resistance?

If you require this to work at 3 volts, and the 8.5v supply is not an option, then you might need to do DC-to-DC conversion. Otherwise, this is how I interpret what you are saying:

Use the 3v supply to drive the switch and CD4027 and have the Q or Q-bar output drive a transistor that drives the relay coil via a 5v supply driven by the 8.5v source. In other words, the 8.5v source feeds a 5v regulator that drives the relay coil via the "open-collector" transistor circuit. This, of course, is assuming that the 8.5v supply can deliver enough current to drive the relay coil.

If it must all work from the 3v supply, then, again, the 3v supply must be capable of driving the relay coil, whether that's directly, or via a DC-to-DC converter.

That attached schematic is merely to present the logic. I'll leave the voltage levels to you.
#3 21660568 04 Jan 2012 02:37

Steve Lawson Steve Lawson

Anonymous

Post #3
21660568 04 Jan 2012 02:37

Oops, got that circuit wrong. The attached is the corrected version...
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#4 21660569 04 Jan 2012 08:17

RAJU MUKTAWAR RAJU MUKTAWAR

Anonymous

Post #4
21660569 04 Jan 2012 08:17

Thanks for your reply,and the circuit.
Can I swap the switch and R1 ?As I want the switch from the positive edge.

Can I give 3v to the IC and 8.5v to relay through 7805?and the other end to the transistor?

And how to filter the debounces at the CLK input of the IC4027?
#5 21660570 04 Jan 2012 13:41

Steve Lawson Steve Lawson

Anonymous

Post #5
21660570 04 Jan 2012 13:41

Oops, my brain was AWOL--probably too much "holiday cheer"! I attached a new circuit. The Schmitt trigger (74AC14) conditions the push button signal into a sharp edged waveform to prevent excessive current draw in the 4027 (otherwise, as the capacitor (C1) discharges through R1, the 4027 would be driven into it's active region). I put two schmitt trigger inverters in series to provide a rising edge to the clock input upon "press" rather than "release". Adjust as needed. You can try it without the schmitt trigger, but I wouldn't recommend it.

The only way around having to use the input conditioning circuit (R1, C1, IC2a, IC2b) is to use a double pole, single throw push button and wire it to the S/R inputs of the 4027.
#6 21660571 04 Jan 2012 14:16

RAJU MUKTAWAR RAJU MUKTAWAR

Anonymous

Post #6
21660571 04 Jan 2012 14:16

Hi, Thank you very much for this circuit.
I tried the previous circuit and it worked very good,but there are two problems.
I think the one would be solved by the new circuit.
The second is,
I am using this circuit as add on circuit to SONY PS2 video game console.
I am taking the 8.5v and 3v supply from the joystick controller of the video game.
But when I operate this circuit,the joystick controller is disconnecting from the video game console for a fraction of seconds and connects again.
Can you give me any suggestion for this problem?
#7 21660572 04 Jan 2012 14:29

Steve Lawson Steve Lawson

Anonymous

Post #7
21660572 04 Jan 2012 14:29

What do you mean by "disconnecting"? Is there some physical connection that's breaking, or is some voltage dropping momentarily, or what?! It would be really nice if you would provide more detailed descriptions. Like, for instance, it would have been really cool if you had mentioned all that stuff about the joystick controller and such in your first post!

And, you still haven't given me any specs on that relay.
#8 21660573 04 Jan 2012 14:49

RAJU MUKTAWAR RAJU MUKTAWAR

Anonymous

Post #8
21660573 04 Jan 2012 14:49

I am using 2 relays paralleled by a BC547 transistor.
The relay is a 5v DPDT relay 167 ohms
http://www.datasheetcatalog.org/datasheets2/15/150649_2.pdf
I am unable to explain about the problem, but the joystick is based on a micro processor and some sort of disturbance is happening while pressing the push to on switch. I think there is no problem with our circuit.
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#9 21660574 04 Jan 2012 15:12

Steve Lawson Steve Lawson

Anonymous

Post #9
21660574 04 Jan 2012 15:12

OK, here's how you provide detail. Rather than saying "some sort of disturbance", tell me how you came to that conclusion, i.e. what is happening, exactly--what does the disturbance look like--what _tells_ you there is a "disturbance"?

But, I'll offer this (a shot in the dark): perhaps the relay is drawing more current than the 8.5v supply can deliver. Try using an external power supply to supply the 8.5 volts and see if the "disturbance" goes away.

Also, the BC547 has a typical beta of 120 at 100ma. The two relays will be drawing something like 60ma. 60/120 = 0.5ma. That's probably more than the 4027 can deliver. So, two things:
# The relay might not be getting enough drive current.
# The 3 volt supply might be getting pulled down by the transistor (unable to supply that .5 ma base current.
# If you didn't already, try putting a 01nF cap across the VDD and GND pins on both the 4027 and the 74AC14.
#10 21660575 04 Jan 2012 15:13

Steve Lawson Steve Lawson

Anonymous

Post #10
21660575 04 Jan 2012 15:13

Oops, that should be "10nf" across the VDD and GND pins.
#11 21660576 04 Jan 2012 15:29

RAJU MUKTAWAR RAJU MUKTAWAR

Anonymous

Post #11
21660576 04 Jan 2012 15:29

I tried this circuit with only an LED.
The same problem occurred.
#12 21660577 04 Jan 2012 15:30

RAJU MUKTAWAR RAJU MUKTAWAR

Anonymous

Post #12
21660577 04 Jan 2012 15:30

Thank you,will try the circuit tomorrow and check the results.
#13 21660578 04 Jan 2012 15:48

Steve Lawson Steve Lawson

Anonymous

Post #13
21660578 04 Jan 2012 15:48

The LED might, also, be drawing too much current. You need to find out how much current each of these supplies can handle. Try using a potentiometer with a current meter and volt meter to see how much current you can draw before the voltage starts to drop.
Be sure to start with a high resistance and then slowly lower it until the voltage starts to dip. Make note of that current--it's the maximum. Also, see if you can cause your "disturbance" with the pot--cautiously (you probably don't want to burn out either of these supplies).
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#14 21660579 05 Jan 2012 02:09

RAJU MUKTAWAR RAJU MUKTAWAR

Anonymous

Post #14
21660579 05 Jan 2012 02:09

Hi, The circuit is perfectly alright.
There was a component fault for the disturbance.
Now the circuit is perfectly ok without 74ac14 IC.
I am very thankful to you for your guidance and for the circuit.
My name is RAJU and I am from INDIA.
How can I contact you in future for ......
#15 21660580 05 Jan 2012 02:17

Steve Lawson Steve Lawson

Anonymous

Post #15
21660580 05 Jan 2012 02:17

Great. I'm glad I was of help. Future contact via EEWeb would be fine.
#16 21660581 03 Jul 2012 21:47

RAJU MUKTAWAR RAJU MUKTAWAR

Anonymous

Post #16
21660581 03 Jul 2012 21:47

Hi, How can I use 3v relay at 4027 output instead of using a 5v relay?
I want to use the whole circuit to run on 3v. What components shall I use to drive a two 3v relays at 4027 output.
#17 21660582 03 Jul 2012 21:52

RAJU MUKTAWAR RAJU MUKTAWAR

Anonymous

Post #17
21660582 03 Jul 2012 21:52

When I operate the switch SW1, the 3v is getting short and some disturbance is occuring to the circuit from where I am taking the 3v supply. But this circuit is working perfectly.
#18 21660583 04 Jul 2012 00:31

Chris Tremlett Chris Tremlett

Anonymous

Post #18
21660583 04 Jul 2012 00:31

It is not so difficult to which you think, i think you should try to make this , you'll be success in that
#19 21660584 05 Jul 2012 09:21

Steve Lawson Steve Lawson

Anonymous

Post #19
21660584 05 Jul 2012 09:21

to run the whole thing on 3 volts (with a 3V relay) simply drive the relay from the 3 volt supply. Be sure, though, to isolate the relay supply line from the rest of the 3V supply feeds (i.e. run separate lines to the source, so the relay current isn't travelling down the same line that supplys the logic ICs. Also, place 10nF caps across the supply lines on the ICs (as close to the supply terminals as possible).
#20 21660585 05 Jul 2012 11:58

RAJU MUKTAWAR RAJU MUKTAWAR

Anonymous

Post #20
21660585 05 Jul 2012 11:58

Hi, Can you tell me the transistor number to drive two 3v relays of 65 ohms each.from the output of 4027
#21 21660586 06 Jul 2012 19:00

Steve Lawson Steve Lawson

Anonymous

Post #21
21660586 06 Jul 2012 19:00

PN2222A
#22 21660587 10 Jul 2012 23:28

RAJU MUKTAWAR RAJU MUKTAWAR

Anonymous

Post #22
21660587 10 Jul 2012 23:28

Hi, STEVE
The circuit is working correctly but I am using two circuits for the two fi=lip flops of 4027.
I am driving 2 relays of 3v 45 ohms in parallel through BC 337 to the 4027 out put.
But when one of the 4027 is working,the other is not working.
If the first part of 4027 is on activate position,the next part of 4027 is not working,and if I deactivate the first and operate the second half of the 4027, then the second is working.But both flipflops of the 4027 are not working simultaneously.
What would be the problem?
#23 21660588 11 Jul 2012 03:41

Steve Lawson Steve Lawson

Anonymous

Post #23
21660588 11 Jul 2012 03:41

OK, shot in the dark: make sure all unused inputs are tied either low or high. And it wouldn't hurt to add a 10nF ceramic cap across all Vdd/Vss pins. Beyond that, you will have to be more specific than "not working".

What is it not doing? What is the failure mode, exactly?
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Topic summary

✨ The discussion focuses on designing a push ON/OFF latch circuit using the CD4027 flip-flop IC to drive a 5V relay with a 3V input control voltage. Key challenges include powering the 5V relay coil when the control logic operates at 3V and managing relay drive current. Solutions involve using the 3V supply for the CD4027 and push button input, while employing a transistor (e.g., BC547 or BC337) to switch the relay coil powered by a regulated 5V supply derived from an 8.5V source via a 7805 voltage regulator. Debounce filtering at the CD4027 clock input is addressed by adding an RC network and Schmitt trigger inverters (74AC14) to produce clean rising edges and prevent multiple triggering. Users report issues with supply disturbances affecting a connected Sony PS2 joystick controller, likely due to current draw or component faults; suggestions include isolating relay supply lines, adding decoupling capacitors (10nF) across IC power pins, and verifying power supply current capacity. For running the entire circuit at 3V with 3V relays, it is recommended to isolate relay supply lines from logic supply lines and use suitable transistors like PN2222A or BC337 to drive relays with coil resistances around 45-65 ohms. Problems with simultaneous operation of both halves of the CD4027 are attributed to improper input handling and insufficient decoupling. Overall, the circuit is feasible with proper power management, input conditioning, and component selection.
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CD4027 Push ON/OFF Latch Circuit to Drive 5V Relay Using 3V Input Voltage

FAQ

TL;DR: Build a push-on/off latch with a CD4027 at 3 V that drives a relay via a transistor; a 5 V, 167 Ω coil draws ~30 mA. “The Schmitt trigger (74AC14) conditions the push button signal.” [Elektroda, Steve Lawson, post #21660570]

Why it matters: This FAQ helps hobbyists and repairers wire a reliable 3 V pushbutton latch that safely switches 5 V relays without browning out consoles or controllers.

Quick Facts

Architecture: 3 V powers CD4027; 8.5 V → 7805 → 5 V relay; CD4027 Q drives a transistor (“open-collector” style). [Elektroda, Steve Lawson, post #21660567]
Debounce/edge shaping: R–C into two 74AC14 inverters yields a clean rising-edge clock on button press. [Elektroda, Steve Lawson, post #21660570]
Decoupling: Place 10 nF ceramics across each IC’s VDD/VSS; keep relay supply wiring separate from logic. [Elektroda, Steve Lawson, post #21660584]
Drive device: PN2222A recommended for switching low-voltage relay coils from the 4027 output. [Elektroda, Steve Lawson, post #21660586]
Base-drive check: Two 5 V, 167 Ω relays ≈ 60 mA; with β≈120 at 100 mA, base ≈ 0.5 mA, beyond 4027 comfort. [Elektroda, Steve Lawson, post #21660574]

How do I wire a 3 V push ON/OFF latch to drive a 5 V relay from an 8.5 V source?

Power CD4027 from 3 V. Regulate 8.5 V to 5 V with a 7805 for the relay. Use the 4027 Q (via a base resistor) to drive an NPN transistor that switches the 5 V relay coil. Add a flyback diode across the coil. This keeps logic at 3 V while the relay sees 5 V. [Elektroda, Steve Lawson, post #21660567]

How do I debounce the pushbutton and get a rising-edge clock?

Use an R–C into a 74AC14 Schmitt trigger, then another inverter to restore polarity. This converts the noisy push into a sharp rising edge. As stated, “The Schmitt trigger (74AC14) conditions the push button signal.” [Elektroda, Steve Lawson, post #21660570]

Can the logic run at 3 V while the relay runs at 5 V?

Yes. Run the CD4027 and switch from 3 V. Regulate the 8.5 V rail down to 5 V for the relay. Drive the relay through a transistor from the 4027 output. Ensure the 8.5 V source can supply the relay current. [Elektroda, Steve Lawson, post #21660567]

Which transistor should I use to drive two small 3 V relays from the 4027?

Use a PN2222A with an appropriate base resistor and a diode across each coil. It offers robust switching for low-voltage relay loads from logic-level control. [Elektroda, Steve Lawson, post #21660586]

Pressing SW1 glitches my controller—how do I stop the brownout?

Separate the relay’s 3 V feed from the logic 3 V wiring to prevent shared drops. Add 10 nF decouplers close to each IC. This layout reduces transients that reset sensitive controllers. "Run separate lines to the source" to keep relay current off logic traces. [Elektroda, Steve Lawson, post #21660584]

How much current do my relays need, and what base current is required?

Example: two 5 V, 167 Ω coils draw about 60 mA total. With a BC547 at β≈120 at 100 mA, base current needs about 0.5 mA. The 4027 at 3 V may not source that reliably, so use proper buffering. [Elektroda, Steve Lawson, post #21660574]

Can I swap the switch and R1 to trigger on the positive edge?

Use two Schmitt inverters after the R–C to produce a press-detected rising edge at the 4027 clock. This achieves your positive-edge requirement without rewiring the basic R–C. [Elektroda, Steve Lawson, post #21660570]

How do I run the entire setup from 3 V using 3 V relays?

Drive the 3 V relays from the 3 V rail through a transistor, not directly from the 4027. Route separate supply lines for relay and logic, and place 10 nF decouplers at the IC pins. [Elektroda, Steve Lawson, post #21660584]

Why won’t both halves of my CD4027 work at the same time?

Tie all unused inputs definitively high or low, and decouple each supply pin pair with 10 nF. Floating inputs or poor decoupling can prevent the second flip-flop from toggling. Describe exact failure modes during debugging. [Elektroda, Steve Lawson, post #21660588]

My circuit works even without the 74AC14—what changed?

A faulty component caused the earlier disturbance. After replacement, the latch worked cleanly without the Schmitt trigger in that build. Component health can mimic debounce issues. [Elektroda, RAJU MUKTAWAR, post #21660579]

How can I test whether my 3 V or 8.5 V rails can supply enough current?

Use a potentiometer as a variable load with volt and current meters. Start at high resistance, lower slowly, and note the current when voltage sags. Stay within safe limits to avoid damage. [Elektroda, Steve Lawson, post #21660578]

What does the CD4027 do in this latch?

The CD4027 stores the ON/OFF state. A conditioned button pulse clocks it, toggling Q, which then drives the transistorized relay stage. The Schmitt-triggered edge prevents active-region oscillation. [Elektroda, Steve Lawson, post #21660570]

What relay current should I expect with two 3 V, 65 Ω coils in parallel?

Each coil draws about 46 mA at 3 V. Two in parallel need roughly 92 mA. Size the transistor and supply wiring accordingly, and add flyback diodes. [Elektroda, RAJU MUKTAWAR, post #21660585]

Is a 74AC14 mandatory for debounce?

It’s recommended to avoid slow edges that increase IC current. However, a clean switch and healthy components can work without it in some builds. Use it when reliability is critical. [Elektroda, Steve Lawson, post #21660570]

Any quick build steps for the 3 V to 5 V relay latch?

Power CD4027 and button from 3 V; add 10 nF decouplers.
Condition the button with R–C into two 74AC14 inverters.
Use Q to drive an NPN that switches a 5 V relay fed by 8.5 V→7805; add a flyback diode. [Elektroda, Steve Lawson, post #21660567]

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