# Replacing an NE555 with a 74HC123D

I want to replace an NE555 with a 74HC123D in a battery powered device. It is used to clock a CD4017, for which I need a pulsed signal of 46Hz.
I read that a 74HC123D needs less power than a 555. How can I generate that signal with it?

My controller (which is also in the circuit) needs less than 1µA and a timer like the 555 with 100µ would be quite an amount of additional current.

• Why do you not have any acceptance critieria? Tolerance on f, Vdd, I max. How is easy or hard depending on specs. tinyurl.com/tv68ah9 Jan 29 '20 at 6:01
• I recall from a section in the 2nd edition of The Art of Electronics, page 377, a discussion of a micropower switching supply that has a quiescent current of $1\:\mu\text{A}$. To achieve this it uses a 2N6028 PUJT is used. If you look at page 968, you'll see that the 2N6028 is "specified for peak currents as small as $0.1\:\mu\text{A}$ and can thus be operated as an oscillator at less than $1\:\mu\text{A}$." Figure 7.6 of the 3rd edition on page 428 also shows this circuit.
– jonk
Jan 29 '20 at 7:08
• At 5V a CD40106 (hex Schmiit inverter draws 0.02 uA typical, 1 uA max at 25C. See datasheet for other conditions. Using a 10M feedback resistor and around 1nF cap an iscillator using one gate will draw around an additional 0.1 uA for the capacitor drivng. Jan 29 '20 at 7:35
• By controller do you mean a microcontroller? If it's already in your circuit, and happens to have an unused pin with PWM, you can save a 555/123. The component that consumes the least power is the one you leave in your drawer.
– Nyos
Jan 30 '20 at 0:26

Because the 123 has both positive and negative trigger inputs, it is possible to feed the output back to the input and create an oscillator. However, it is not as simple as it might seem because the timing capacitor needs time to discharge between cycles. The result is an oscillator circuit that is less accurate/precise/stable than a 555 circuit.

Your question is comparing apples and oranges. HC stands for High-speed CMOS, while a 555 (without any prefix) is assumed to be the original, bipolar design. If you compare the datasheet static or standby current values for a bipolar NE555 and a CMOS LMC555N, you will see that the standby current values are much lower for the CMOS part.

Checking some datasheets, I see that the HC123 has a max operating current of 80 mA, while the LMC555 comes in at around 100 uA typical and 250 uA max. at 5 V. Given the difference in part size and cost, and circuit complexity, how badly do you need the lower operating current?

• quite badly my controller needs less than 1µA which is also in the circuit and a timer like the 555 with 100µ would be quite an amount. According to my datasheet the 123 needs less asset.conrad.com/media10/add/160267/c1/-/en/001116055DS00/… Jan 29 '20 at 5:33
• This information should be in the question. Jan 29 '20 at 6:16
• As you no doubt know, 555 bistable circuits usually implement a charge via two series R's, discharge via one topology, which leads to asymmetric mark space. This can be largely overcome with extra circuitry. Jan 30 '20 at 1:45

A 74HC123 / 74HCT123 datasheet here draws 8 uA max at Vcc=5.5V at 25C. That's for the IC itself - current in the timing resistor will add to that.

A lower current option is to build an oscillator from a single Schmitt trigger gate. Supply current is typically 0.02 uA and timing components add perhaps ~= 0.5 uA - less if care is taken.

At 5V a CD40106 (hex Schmiit inverter draws 0.02 uA typical, 1 uA max at 25C.
See datasheet for other conditions.

Using a 10M feedback resistor and around 2nF cap an oscillator using one gate will draw around an additional 0.5 uA or less for the capacitor driving. Ground the other gate inputs and leave the outputs open.

Icap is i = V/R = (Vhigh - Vschmitt low)/10M when the capacitor starts to charge, falling to ((Vhigh - Vschmitt high)/10M just before the gate switches. As the low and high thresholds are in the order of Vcc/3 and 2.Vcc/3 the charge current with a 10M resistor should vary from about 2/3 x Vcc/10M to 1/3 x Vcc / 10M as the capacitor charges or discharges.

A larger resistor than 10M may be used if due attention is paid to leakage paths and environmental cleanliness.

The gate internal currents may change as Vin = Vcap transitions exponentially between low and high states and this is not well covered by the data sheet. Experimentation (or simulation) will be required to confirm current drain, but it is liable to be low.

simulate this circuit – Schematic created using CircuitLab

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how stable is this with changing temperature when using it ouside?

At exactly 25 degrees C it's perfectly temperature stable.

If you have more details of what you wish to achieve you should place them in your question. Otherwise we go around and around and ... in circles and waste your time and ours.

What stability spec do you want?
What is your expected temperature range?

Next will you ask "and what about voltage variation", and humidity, and .... ? Resistor and capacitor variation with temperature may matter.
Device threshold variation with temperature MAY be in the spec sheet - did you check?
We can try to help, but knowing what you want is essential.

• thanks, how stable is this with changing temperature when using it ouside? Jan 29 '20 at 16:06
• Over what temperature range? Jan 29 '20 at 20:11
• @user3435167 At eactly 25 degrees C it's perfectly temperature stable. If you have more details of what you wish to achieve you should place them in your question. Otherwise we go around and around and ... in circles and waste your time and ours. What stability spec do you want? What is your expected temperature range? Next will you ask "and what about voltage variation", and humidity, and .... ? Resistor and capacitor variation with temperature may matter. Device threshold variation with temperature MAY be in the spec sheet - did you check? Jan 29 '20 at 20:54
• I tested a CD40106 and a CD4093. At 5V they both drew 26uA. At 3.3V the CD40106 drew 1.2uA and the CD4093 drew 1.6uA. Reducing resistance from 10M to 1M only increased current by ~0.8uA at 3.3V and 2uA at 5V, which suggests the major contribution is from FET conduction. Jan 29 '20 at 21:01
• This is real components on the bench, all other inputs held at Vdd/Vss. BTW hysteresis was much less than I expected. At 5V the CD40106 triangle wave was only 0.6Vpp (12% of Vdd). So much for specs! Jan 29 '20 at 21:19