one digit BCD down counter starting at 0100

Thread Starter


Joined Nov 10, 2013
I am using DC circuit.
The result need to be start with 4 (4>3>2>1>0>9>8>7>6>5>4) and show by a 7-seg. LED. and down count for each second. No switches allowed

i know i need to use a:
74LS47 as 7 seg. decoder,
74LS190 as decade down counter and
NE555N as timer.

and my lecturer just give me a hints:
T=5RC (time equal to 5 times of resistance and capacitance)
so I'm planning to use a R10kΩ and 0.1uF cap to delay for 0100 starting.(74LS190 start at 9, I need T=5((9>8>7>6>5>4)) for starting the counting at 4. So, 5=5RC , 5=5(10kΩ)(0.1uF))

i am not going to let you tell me the whole construction of the circuit.

what i want to know is:
1. what is the truth table of 74LS190 when down counting?
2. how the (T=5RC) actually works? (does my plan works? If it does,)
2.1 where the RC circuit would be connected?
3. what is threshold and trigger at NE555N used to be?
4. is the pcb contain more component that i need to use?
5. is there some software to simulate a circuit? where should i download?

My english is so bad and reading datasheet is a nightmare. There should be more question but i don't know how to express. Sorry about that. If something confuse you or not enough evidence, ask me. And i would answer as detail as i can afford.
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Joined Jun 26, 2012
Since the 74LS190 is a PRESETTABLE counter, why not wire the D-C-B-A inputs as 0-1-0-0 and use a short RC (a few microseconds) on the LOAD/ pin? On power up, the LOAD/ pin will be low awhile and start the counter at 4. T=5RC gets the capacitor nearly fully charged but the LOAD/ timing will be shorter because of the LSTTL voltage thresholds, maybe 1.1 RC. That will initialize the counter.

Use the 555 to generate 1 second pulses to clock the counter.
As far as the 555 timing there are charts and formulas in the datasheet that should not be too hard to understand. There are lots of 555 circuits here at AAC as well. The basic 'astable' oscillator is what you want. Connect the 555 output (pin 3) to the CLK input of the counter. Connect CTEN low and D/U high to enable counting down. The data is on the Q outputs.

For approx. one second, Fig 17 of the TI datasheet shows:
RA (from Vcc to pin 7) + 2RB (between pin 7 and 6) as 100K with a capacitor of 10uf to generate a 1 second period for counting.

Good luck.
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