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[SOLVED] Digital Clock with Voltage Regulator: Weird Behavior

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fishcake

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I'm really new with electronics, so bear with me if I'm doing something horribly wrong. :p

For my first project, I'm developing a digital clock using a bunch of ICs (TTL and CMOS) and a 1MHz oscillator. I don't have the schematic for the circuit right now (will update this post if it is needed), but it's basically like this: 1MHz oscillator provides 1MHz pulse (duh), which is then converted to 1Hz using 3 74390 (each divides by 100). The pulse is then used to drive 4 74LS90: 2 for the seconds and 2 for the minutes (I ran out of breadboard space for the hours :-(). Each decade counter is then connected to a 4511 (BCD to 7-segment decoder), which is then connected to a common cathode 7-segment display.

While working on this circuit, I temporarily used Arduino to supply a 5v voltage. Everything worked fine. Awesome. But when I switched to using a 9v battery converted to 5v using 7805 voltage regulator (with 47uF capacitor connected to input and 10uF capacitor connected to output), weird things start happening. One of the 2 "seconds" 7-segment display randomly blanks out (i.e. it doesn't light up) regularly. Like in 5 seconds, it will blank out once . Sometimes, both digits blank out. And to make things worse, after some time, the clock stops ticking... What the hell did I do wrong? :p
 

sky_123

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Make sure there is a very short-leaded capacitor of 100nF across the power rails close to each logic IC, similar to the image here. Put on on the output of the regulator too. Also, double-check with a voltmeter that the supply out of the regulator is 5V. Maybe the 9V battery can't handle the load.
cap.jpg
 

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betwixt

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My guess would be that the battery can't supply the load current. If it dips below about 8V the regulator will have insufficient voltage overhead to function and the 5V supply will drop out. As you are using direct drive to the LEDs, the current demand could be quite high when some digits are displayed, possibly several hundred mA could be drawn from the battery. MEasure the battery voltage,if it gets close to 8V, that's where the trouble lies.

Brian.
 

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fishcake

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Yes, my battery does have a voltage lower than 8V. It was around 6V yesterday, and there's only 3V left today. And the output of the voltage regulator is indeed lower than 5V. Does this means that my circuit, having a lot of ICs and LEDs (or any circuit that has a lot of ICs), will only work definitely when the battery's supply voltage is higher than 8V, and is undefined when vice versa? Is there a way to make the circuit work even when the battery can't handle the load (or is it stupid to even ask such a question? :oops:)?
 

sky_123

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LED displays by their nature tend to consume (relatively speaking compared to the rest of your circuit) a large amount of current.
You could try a different regulator (lower voltage dropout) that lets you stretch the time a little bit longer, or use a mains adapter.
Even if you use a different regulator, it won't make such an impact to the life if you're using the small PP3 type of 9V battery. The circuit
you have developed really needs to be powered by a mains adapter. All commercial LED display clocks also require a mains adapter,
for the same reason.
An LCD display can be run by battery, but it is very difficult to create a clock using an LCD without using a lot more complicated circuit.
 

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betwixt

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sky_123 is absolutely right. You problem is compunded by the regulator itself which needs at least 8V at it's input to maintain 5V at it's output. Below 8V it becomes somewhat unpredictable and can even oscillate it's output voltage up and down.

Commercial LED displays usually use a system called multiplexing where they use a trick of eyesight to reduce wiring and current consumption. Our eyes can't see very fast flashes so rather than drive all the LEDs simultaeously like you do, each digit is switched on and off again in sequence, so fast that they all appear to be on at once. Electrically, because only one has power to it at a time, the current consumption is much less. Unfortunately, your driver circuit doesn't lend itself to multiplexing easily although with care you could sequence the 4511s so all but one is disabled at any time.

Brian.
 

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fishcake

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Thank you for the replies! :-D I watched a video about battery capacity, and there's a lot of stuff I need to know to design a circuit that can be powered by a battery. Well, screw that for now! I'm buying myself a mains adapter for this project. :razz: And I'll keep multiplexing in mind for my future projects that involve lots of LEDs.
 

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