the output frequency of the 74ls294 seems too high?

i put a ne555 timer in astable mode. the variant of 555 timer is ne555. the output of the 555 timer is the input to a 74ls294 frequency divider. the output of the 74ls294 frequency divider turns a led on and off. i used a timer to time the on period and off period. the led is on for 9 seconds and off for 9 seconds. E pin is connected to positive terminal. A pin is connected to positive terminal. B pin is connected to positive terminal. C pin is connected to negative terminal. D pin is connected to positive terminal. the 74ls294 frequency divider is suppose to divide the input frequency by 2 to the 27th power. i calculated an input frequency of 7.4 million hertz. the ne555 timer is rated at a maximum frequency of 100 kilohertz. the capacitor is rated at 10 microfarads. the first resistor is rated at 1000 ohms. the second resistor is rated at 330 ohms. i calculated the output frequency of the 555 timer is 61 hertz.i connected the clear pin of the 74ls294 to the positive terminal. there are two clock pins. the output of the 555 timer is connected to the second clock pin. the first clock pin is connected to the negative terminal.

this is a schematic of my circuit.


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why am i getting an output frequency much higher than 100 kilohertz?

A number of problems there:
1. The supply voltage is too low for both ICs to operate properly
2. the resistor values around the NE555 are much lower than the manufacturer recommends
3. You have not told us which output of the 74LS294 you are using, it has four outputs!

If the supply was adequate, the NE555 would oscillate at about 62Hz.
So the outputs from the LS294 would be:
Q = 0.000000461 Hz so it would turn the LED on or off about once every 25 hours.
TP1 = 1.9375Hz, roughly every two seconds
TP2 = 0.0075Hz, roughly every 2 minutes and 10 seconds
TP3 = 0.00006Hz, roughly every 4 hours 35 minutes.

Brian.

betwixt said:

A number of problems there:
1. The supply voltage is too low for both ICs to operate properly
2. the resistor values around the NE555 are much lower than the manufacturer recommends
3. You have not told us which output of the 74LS294 you are using, it has four outputs!

If the supply was adequate, the NE555 would oscillate at about 62Hz.
So the outputs from the LS294 would be:
Q = 0.000000461 Hz so it would turn the LED on or off about once every 25 hours.
TP1 = 1.9375Hz, roughly every two seconds
TP2 = 0.0075Hz, roughly every 2 minutes and 10 seconds
TP3 = 0.00006Hz, roughly every 4 hours 35 minutes.

Brian.

Click to expand...

i use the q output.

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betwixt said:

A number of problems there:
1. The supply voltage is too low for both ICs to operate properly
2. the resistor values around the NE555 are much lower than the manufacturer recommends
3. You have not told us which output of the 74LS294 you are using, it has four outputs!

If the supply was adequate, the NE555 would oscillate at about 62Hz.
So the outputs from the LS294 would be:
Q = 0.000000461 Hz so it would turn the LED on or off about once every 25 hours.
TP1 = 1.9375Hz, roughly every two seconds
TP2 = 0.0075Hz, roughly every 2 minutes and 10 seconds
TP3 = 0.00006Hz, roughly every 4 hours 35 minutes.

Brian.

Click to expand...

what values of resistors does the manufacturer recommend?

Hi,

Good to see a schematic.
But please show a complete schematic, textual descriptions are hard to read.
And if possible follow general rules like:
* GND on bottom of sheet, VCC at top of sheet.
* use GND and VCC symbols
* signal flow from left to right

No input of a logic IC must be left floating. Connect each input to proper logic levels.

Klaus

what values of resistors does the manufacturer recommend?

Click to expand...

Check the data sheet!
Generally they recommend values no lower that 1K and no greater than about 3M. Using lower values and hence larger capacitor value just wastes current and increases power dissipation. For example in your circuit you can get almost the same timing with 22K, 68K and 0.15uF but with a fraction of the current being used.

Brian.

how do i provide the circuit with five volts? the recommended operating supply voltage is 5 volts of both ne555 timer and 74ls294.

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according to the data sheets.

You finally realized 5V (> 4.5) supply requirement. Suggest to use 3V capable CMOS variant of both chips, or boost converter. The circuit application isn't clear, thus I can only guess about suitable solutions. Presently I don't even understand, why these chips have been chosen to blink a LED.

FvM said:

You finally realized 5V (> 4.5) supply requirement. Suggest to use 3V capable CMOS variant of both chips, or boost converter. The circuit application isn't clear, thus I can only guess about suitable solutions. Presently I don't even understand, why these chips have been chosen to blink a LED.

Click to expand...

i am trying to make sure the 555 timer in astable mode works as intended.

Your LED is upside down. A red one is 2.0V. The old fashioned 74LS274 output goes low to about 0.4V. So with a +5V supply, your 1k current-limiting resistor will have 2.6V across it and the LED current will be only 2.6mA and look dim.