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By using our site, you acknowledge that you have read and understand our Cookie PolicyPrivacy Policyand our Terms of Service. I’m a complete amateur, I’ve probably gotten a million things wrong with this already.

I’m using an SN74HC dual j-k” flip-flop 7h4c109. I’m trying to toggle the led forgot to add a resistor in the diagram, ah, also forgot to add a line from 5V to Vcc.

When the circuit is powered, the led turns on but does not toggle when the momentary switch is pressed and released. There are exceptions, with some circuits incorporating a built-in pull-up or pull-down resistor, but unless the data sheet mentions these, assume they don’t exist.

74HC109 DIP-16 J/K flip-flop

Leaving unused pins floating particularly with CMOS can produce amazingly intermittent, bizarre and maddening errors. And 47hc109 think that you can ignore the inputs of an unused part of the circuit, such as the unused flip-flop in your 74HC Don’t even think about it.


Sometimes that works, and sometimes it doesn’t. Tie them all high or low. Caps are cheap and don’t take up much space, but the problems arising from not enough of them datasheer be hard to diagnose.

Connect the cap directly to the sockets nearest the IC power and ground pins. On the one hand, it will try to draw too much current in the on state, and will also clamp the output to a low voltage which the input will not recognize as high.

74HC Datasheet(PDF) – NXP Semiconductors

In this case, a ohm resistor, combined with the LED voltage and the limited high output drive of the flip-flop will give you about 6 or 7 mA LED current when on. Just to show what I’m talking about, once you’ve got the circuit working, put an LED directly to 47hc109 on the flip-flop clock input, and watch what happens.

In this case, it will show up as intermittent operation of the flip-flop, depending on whether or not it got an even or odd number of contacts when you press the switch.

For more complex circuits using counters or state machines you can get the most amazing errors. You are clocking a flip-flop directly from a switch that has not been de-bounced. You need a de-bouncing circuit to feed the flip-flop’s clock input. As others have suggested, this could be a simple R-C network, or as a better option, use an SPDT change-over switch and a set-reset SR flip-flop as a de-bouncer.


A common way to do this is with cross-connected NAND gates. By clicking “Post Your Answer”, you acknowledge that you have read our updated terms of serviceprivacy policy and cookie policyand that your continued use of the website is subject to these policies.

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Maximum Low-Level input voltage with my Vcc at 5V is about 1. LED does not light up, I’ve done something wrong again haven’t I? Just pull it down to 0V with a single resistor!

74HC 데이터시트(PDF) – NXP Semiconductors

LED lights up but does not toggle. Please build the following circuit: WhatRoughBeast 49k 2 28 Always connect the GND pin. Never leave it unconnected, even if the circuit appears to work. John Honniball 2, 6 9.

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