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I am designing a small PCB which I want to power using a CR2032 coin cell. The circuit will include a 555 timer, a 40xx TTL and a few small LED's. I chose both IC's with a minimum supply voltage of 2V to make it work with the coin cell.

What are the conventions with powering circuits with a coin cell? Do I need to use a (linear) power regulator for this or can the battery be hooked up straight away?

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    \$\begingroup\$ How sensitive is the functionality of your circuit to the supply voltage? \$\endgroup\$ – Dave Tweed Oct 2 '16 at 14:49
  • \$\begingroup\$ @DaveTweed Only the 555 might find problems in power supply drops, as it changes the triggering threshold. \$\endgroup\$ – Ferry Oct 2 '16 at 17:27
  • \$\begingroup\$ Actually, that isn't true. The 555 is a ratiometric device -- both the charge/discharge rate and the two triggering thresholds are proportional to the supply voltage, so any changes in the supply voltage do NOT change the timing parameters. Of course, this ignores secondary effects such as comparator offsets and input bias currents, so there is some small effect on the timing in practice. However, you never said how critical this would be to the correct functioning of your circuit, whatever that might be. \$\endgroup\$ – Dave Tweed Oct 3 '16 at 11:22
  • \$\begingroup\$ I am planning to make a small electronic dice PCB as a practice for my PCB design skills and aim to realize this without a microcontroller. I understand that the 555 internal components work proportional. My main point was that external charged capacitors which influence the triggering timing do not directly scale with supply variations, but this might be negligible. \$\endgroup\$ – Ferry Oct 3 '16 at 15:11
  • \$\begingroup\$ This "external charge" comes from somewhere other than the coin cell? \$\endgroup\$ – Dave Tweed Oct 3 '16 at 21:20
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I am designing a small PCB which I want to power using a CR2032 coin cell.

OK. You've got 3 V when new.

The circuit will include a 555 timer, ...

Yellow warning lamp! As far as I remember the original 555 timers draw a large current at the instant of output change. Consider the use of the CMOS version.

... a 40xx TTL ...

40xx is CMOS not TTL. CMOS should work OK but check the datasheets for minimum working voltage. TTL works in a voltage range around 5 V.

... and a few small LED's.

Two things to watch for:

  • Watch the ESR (equivelant series resistance) of your CR2032. The more current you pull the more the terminal voltage will drop.
  • Bearing the above in mind, you need enough voltage (at full load) to power the LED and whatever voltage drop is across the transistor switch.

I chose both IC's with a minimum supply voltage of 2V to make it work with the coin cell.

Sounds good.

What are the conventions with powering circuits with a coin cell? Do I need to use a (linear) power regulator for this or can the battery be hooked up straight away?

Hook it up directly. You won't over-volt and any kind of regulator will cost power.

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    \$\begingroup\$ Thanks for the info! Regarding the 555 timer: I already chose the TLC555CDR, which is a TI CMOS, so this should work out fine. I will also keep an eye out for the max current with the LED's. \$\endgroup\$ – Ferry Oct 2 '16 at 15:05
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Chips may operate from 2V to 6V or whatever, but they will behave differently. At lower voltages the speed goes down and the drive capability may drop dramatically. 2.0V is an appropriate cutoff voltage for a CR2032.

4000 series CMOS logic is only specified down to 4.5V. 74HC and some newer families work much better at lower voltages. Some newer families of logic may be marginal at the voltage of a fresh CR2032 (too high), others are specified from 0.8 to 3.3V so are ideal.

CR2032 batteries are specified at modest current draw (200uA) and the voltage starts to drop significantly above 1mA or so, so keep the average draw low. If you have sharp current spikes you should add a lot of capacitance. The voltage drop due to internal resistance also increases greatly at low temperatures. You will only get about half the nominal capacity at -10°C and 2mA.

The 555 type circuit timing changes if the supply voltage is not stable within the timing cycle since the charge and discharge thresholds are 2/3 and 1/3 the supply voltage. For example, if you turn an LED on when the output goes high the supply voltage will drop and it will take longer to discharge the timing capacitor than with a stable supply. This can lead to changes in output timing that vary during the battery discharge or with temperature as the internal resistance changes.

Most things you can do with a bit of logic and a 555 can be done better and possibly cheaper with a small microcontroller.

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