i am looking to make a chasing led circuit, now i know i could use a 555 timer and a 4017, but i was wondering if i could use a 555 timer and a few nand gates, because i have a bunch of 7400 chips that im not using, and it would make more scene to use them rather then to go and buy a 6 dollar chip. and if i can use some nand gates could anyone help me out on a schematic. thanks, Kevin
Hi Pccompuman, Sorry, but I do not know of any circuits where the 4017 can be subsituted by NAND gates, although the timer circuit could be replaced by NAND gates (with some resistors and capacitor). When you mention 6 dollar chip, are you referring to the 4017 ? I though this IC is cheap. ARM
Since the 4017 is a decade counter, it is possible to make it from NAND gates. You will need a bunch of them though because you will have to construct T-Type Flip Flops and the decoders. Remember DeMorgan's Laws? You will have to construct a NAND gate RS Latch as the base and build up towards a JK Flip Flop Tie both J and K together to form a T-Type Flip Flop. Since you need to count to 11, the least number of Flip Flops you need is 4 since 2 to the 4th is 15 and 2 to the 3rd is just 8. Tie a direct link from the first Flip Flop to the second and then AND each successive Q from the first to the next and feed it to the next T input (K-Map Shortcut). Since you want to count to 11 and reset, you will have to either K-Map the final count and alter or utilize the shortcut method. For the shortcut method, decode the final count, before reset and then alter the inputs of the T for the specific bits you want to set back / stay at 0. In this case, you want 0000 Final count is 1011 It want to go 1100 So, you examine each bit beginning either with LSB or MSB - I would go with LSB. LSB or bit 0 goes to zero, that is good since you want 0000. The 1st bit also goes to zero, also good. 2nd Bit goes to 1, uh oh. So you want to keep it zero. For this, feed in the decoded "11" signal to the T-Flip Flop designated as bit 2 by inverting the signal and then ANDing it with the signal coming from the other ANDed Q's. 3rd Bit stays at 1. So we have to hammer the stupid thing down to 0. Simply feed the decoded "11" signal to the T of Bit 3's Flip Flop by ORing it with the signal coming from the other ANDed Q's. Now you have a counter that will count to 11 and then reset back to zero. Time to decode the bits. By decode I mean: 0000 is 0 0001 is 1 0010 is 2 0011 is 3... To this, simply AND the bits that are to be one and that is your decoded signal. Note: Make sure to consider any possible repeats you have and provide an Invert AND. Example, you have decoded the signal 3 by so 0011, one AND gate fed with bits 0 and 1. That works for small numbers but what about 0111 = 7? 7 will also pop up on the decode 3 so you will need to do this instead for 3. If w is MSB and z is LSB then you wil follow with this (!x)yz. So if x is high, then your decoder will reject 7 and only process 2. You do not need to waste your time with taking 1111 into consideration because your counter will never count up to 15.
all of that seams to make a alot of scene, cause right now i am deep into digital theory. thanks for the help, its really usefull. yeah i have a hell of a lot of nand gates, and i know that it is the universal gate, so...... the reason why i am looking for this circuit is i have a stop light and i would like for it to be on green, then go to yellow, then go to red, and so on. and plus i always like a challege, and this seams like it will use a lot of nand gates, plus transistors, and relays. thanks for your input, ill come back with my results/questions in a few days.
if you only have three lights, no sense in making a decade counter. you could do with two bits (i think im right? ) save you some effort at least
