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I created a product demo box that completes a circuit once the "push to start" button is pressed. Closing the circuit allows the customer to see my product in action while holding down a button. I want to create a better user experience by maybe doing one of the two options:

  1. swapping out the push-to-start button and replacing it with a PIR motion sensor to close the circuit for about 30 seconds (ie. HC-SR505)
  2. add a timer to the existing setup where the circuit closes for about 30 seconds

What is the easier of the two? What are the steps involved?

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  • \$\begingroup\$ So what is your circuit? Just a battery and a motor or some more complex circuit? \$\endgroup\$
    – Jens
    Commented Apr 16 at 19:28
  • \$\begingroup\$ You are missing an important requirement, standby current draw. Or in simpler terms, how long does the battery need to last if nobody pushes the button? \$\endgroup\$
    – Mattman944
    Commented Apr 16 at 19:40
  • \$\begingroup\$ What experience do you have? Many of us would solve this with a simple microcontroller. If you have never used a microcontroller, a 555 timer might be a better choice. \$\endgroup\$
    – Mattman944
    Commented Apr 16 at 19:41
  • \$\begingroup\$ @Jens - It's a simple circuit which includes a small 1.5V motor, battery source, and the push to start button. The push to start button closes the circuit. \$\endgroup\$
    – Charles
    Commented Apr 17 at 0:45
  • \$\begingroup\$ @Mattman944 I'm not too familiar with electronics and never used a micro controller. If i wanted to go w/ the motion sensor option, I know the larger HC-SR501 PIR motion sensor has an adjustable delay time. If i wanted to go with the push button + 555 timer option, i think i need to revise the button part where the circuit remains closed until the timer is done. I will need to do somre research on how the 555 timer works. How would i connect it to my circuit and how to modify the timer settings.... \$\endgroup\$
    – Charles
    Commented Apr 17 at 0:46

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I will answer for the timer version. If I could get a proper datasheet for the PIR sensor, I might answer for that also.

Since you only have 3V, you need to use a CMOS 555 timer, AKA 7555. All cap values are microfarads.

The current draw will mostly be in the IC, 60 uA typical, 200 uA max. AA batteries have a capacity of about 2000 mAh. In standby, the circuit will run for 2000/0.2 = 10000 hours or over a year. Motors typically draw considerable current, so if people are pushing the button often, it will last much less.

I built the circuit. The current draw in standby mode is 82 uA. My ICM7555 is from Digikey. If you buy from a non-reputable source, you could get a counterfit part and all bets are off.

Note that simple brushed motors are extremely electrically noisy. It can be extremely difficult to get a circuit to work on a solderless breadboard. The noisy right side must be physically separate from the 7555 circuit. The battery and motor wires should connect near the MOSFET and/or flyback diode. I had to add a snubber across the motor to get my circuit to marginally work on a solderless breadboard.

Edit: I built the circuit on a soldered perfboard. It works better, but the electrical noise is still affecting the timing. It is 5% lower when the motor is running.

A 1.5 Volt alkaline battery will fairly quickly go down to 1.2 Volts. This is the value that I attempted to design to.

The circuit works down to 2.4 Volts. But this is really outside the guaranteed range of the MOSFET. MOSFETs designed for 3.3V logic are common, those designed for lower gate drive are not common. For a more reliable circuit, remove R3 and power the left half with 3 AA batteries.

The snubber RC (R6/C5) is optional, I did not install it on the soldered version.

You should use some care in the layout. Since you are inexperienced, you may want to follow mine closely.

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Or, you might look for a delay module. It is unlikely that you will find one that will work on 3V. This would work on a 9V battery. They don't specify the standby current, so I don't know if this will last an hour or a week on a single 9V battery. You will still need a separate 3V for the motor.

This would be a nice module for my stash, so I am going to order one for myself. I will publish the standby current here.

Edit: I received this module, standby current is 15 mA, not suitable for your application.

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https://www.amazon.com/DROK-Controller-Delay-off-0-01s-9999mins-Adjustable/dp/B07RGT5G1X

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  • \$\begingroup\$ Thank you for your schematics. I went down a rabbit hole and started to explore the 555 timer option. I now know that capacitors and resistors change the length of the timer! I do have a breadboard so i will need to replicate your design. Let me start ordering some parts :) \$\endgroup\$
    – Charles
    Commented Apr 17 at 4:25
  • \$\begingroup\$ one question about the battery drain, will your design cause major drain on the battery? This product demo box is sent out to stores and i can't expect the stores to always change batteries. \$\endgroup\$
    – Charles
    Commented Apr 17 at 4:31
  • \$\begingroup\$ @Charles - I added current draw info to my answer. \$\endgroup\$
    – Mattman944
    Commented Apr 17 at 7:41
  • \$\begingroup\$ @Charles - I added the MOSFET and a motor to my solderless breadboard. My motor draws 300 mA @ 3V and generates a tremendous amount of electrical noise. Hopefully, your motor is smaller. I had to add a snubber to get it to work. I may build a proper soldered circuit in the next few days. I will post the results if I do. \$\endgroup\$
    – Mattman944
    Commented Apr 17 at 18:19
  • \$\begingroup\$ My motor's specs are 1.5V≤ 40MA, 3000RPM. I went to my local electronics store and they were able to help me buy most of the parts. I will be on the look out for the electrical noise (not sure what this mean...lol) once i am able to build your schematics on my breadboard. \$\endgroup\$
    – Charles
    Commented Apr 18 at 3:40

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