Electronic Design
And You Thought The 555 Timer Was Dead?

And You Thought The 555 Timer Was Dead?

The 555 timer IC has been reinvented—again. Signetics introduced the enormously popular chip back in 1971, and it quickly became a big hit. Myriad application circuits have used it throughout the decades, proving its unparalleled versatility (see “The 555: Best IC Ever Or Obsolete Anachronism?”). Recently, both Advanced Linear Devices and Semtech have redesigned the chip, improving it in many ways and extending its usefulness—most likely—for years to come.

The ADL7555 and ALD7556 (Fig. 1) from Advanced Linear Devices are single and dual analog IC timers that offer significant benefits over the original bipolar chips. These CMOS devices provide lower power consumption and wider time/frequency ranges in addition to improved timing accuracy and temperature stability. Astable oscillation frequencies can be as high as 2.5 MHz. With the devices’ lower supply current, less leakage current, and improved voltage stability, applications requiring better precision and stability can be implemented.

Each timer has an operating voltage range of 2 to 10 V, a high discharge sinking output current of 80 mA, and the same pin-out as the original chips. Supply current is typically less than 50 µA for the ADL7555.

The new timers use the same basic circuit arrangement as the original 555. Two precision comparators control the state of a set-reset flip-flop. The comparator input voltage divider string is updated with three 200-kΩ resistors instead of the 5-kΩ ones in the original. The divider, along with the lower leakage, allows a wider range of external resistor and capacitor values to be used. Specifically, higher values of resistors and smaller capacitors can be used to achieve the time constants required by the application. The devices can be used in the free-running astable mode and a 50% duty cycle mode. A one-shot mode is also an option. A control input provides for modulation of the operating frequency.

The ALD7555 and ALD7556 are available in eight- or 14-pin small-outline IC (SOIC), plastic dual-inline package (DIP), and seramic DIP (CERDIP) options. Prices are $0.96 for the ALD7555 and $1.57 for the ALD7556 in quantities of 100 or more.

Semtech’s SX8122 is a sub-1-V analog timer implementing the legacy 555 architecture designed for use in low-voltage, battery-operated small appliances. It provides extended battery life via its intrinsic 0.9-V operation without the need for an external boost converter and inductor, saving board space and cost. It also adds low-voltage features to the 555 design with an output that controls a dc motor or another continuous low-voltage element and an output that generates a pulse train to drive a high-brightness LED from a single battery.

This combination of features makes the SX8122 ideal for a wide range of small consumer home and office appliances, such as electric toothbrushes, buzzers, toys, and LED pointers. The device also includes a voltage monitor to enable a simple USB charger for nickel-metal-hydride (NiMH) or nickel-cadmium (NiCd) supplied devices.

The SX8122 timer operates between 0.9 V and 2 V over the full industrial temperature range of –40°C to 85°C, making it ideally suited to run from one AA or AAA alkaline, NiMH, or NiCd battery. The device has a continuous output that can be activated by the press of a button to control either a dc motor or other continuous low-voltage function, as well as a burst output to drive a high-brightness white or green LED or another feature requiring a voltage boost.

Also, the SX8122 is programmable via external resistors and a capacitor, similar to the legacy 555 but at 1 V, and is designed for on/off button debouncing, delayed startup functions, and analog programmable sequencing and time-out in toys and remote control applications. The SX8122 comes in a SOIC eight-pin package and is available in production quantities at $0.36 each in 2,500-piece lots. An evaluation kit is available as well.

Meanwhile, you should start thinking about some neat new 555-type circuits to submit to Electronic Design’s Ideas for Design department. We’d love to see what you can come up with!

Advanced Linear Devices

Semtech Corp.

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