Useless box upgrade

[planedoctor]

I have a useless box I got from http://frivolousengineering.com/?page_id=1568

I would like to make some changes to this and need help with a proper design.

The goal is.

Switch closed starting a 5 second delay then a Vibration motor then runs for 5 seconds then the Motor with arm is then turned on and mechanically opens switch.

This box is powered by 2x AA batteries.

I have designed a timing circuit using TLC555's. My problem is getting enough power switched for the dc motors to run.

I have tried different transistors with no luck do the the low current and low voltage this 3V circuit has. My last hope was getting some relays but I wanted some opinions first.

Should I scrap the whole idea of using these TLC555's and go with something else?

The motors are 1.5-3 Volt DC with .70A max draw

Thanks for the input.

View attachment 67247

 

[DerStrom8]

A transistor is nothing but a switch. Just putting in a resistor will not provide any more power to the device than it would if you were to connect the motor to +V. 3 volts is not much for a motor. I'm assuming you're using a very small (3v) motor, which will not have much torque. I suggest adding in a secondary part of the circuit with a 9-12 volt power supply, and a more powerful motor. Otherwise, you aren't going to get anywhere.

If you were really bent on using a tiny 3v motor, you might be able to pull it off with a gearbox. This will increase the torque, however it will decrease the speed. I recommend just building a second section of the circuit with higher voltage/current capabilities.

Good luck!
Matt

 

[planedoctor]

Thanks for the reply Matt. The first motor I'm trying to drive is drawing a max of .70A. It has a small weight off cent of the output shaft and is just to vibrate the box. It works well in tests for my idea. The other motor is the same and driving an arm via a gearbox.

I was unable to open a TIP42 transistor via the 3V circuit. I even tried using another smaller transistor to power the base. The circuit was tested using led's and then when replaced with the motor there was not enough current flow to turn the motor.

I have tested the circuit by powering it with a 9v battery and running the motor via a TIP42 Transistor and 3V power to just the motor. This worked.

I have thought that the dual power setup was more trouble and there might be an easier way to achieve my result.

The motors and the existing circuit I'm wanting to mod are 3v.

Thoughts?

 

[alec_t]

I was unable to open a TIP42 transistor via the 3V circuit.

Can you post a schematic of that 3V circuit? Also the 9V circuit you found successful?

 

[planedoctor]

Sorry, I used both tip42(pnp) and the tip120(npn) in my testing. Added the cuicuit with the tip120 with circuit driven by 9v and motor by 3v. Just retested this on the breadboard and it works.

I had read the TLC555 had a better sinking current then sourcing so tried each way with no success with only 3v powering board

View attachment 67266

 

[ronv]

Gotta get all the arrows going in the right direction.

 

[planedoctor]

Thanks, changed it.

 

[Mr RB]

They used to sell these in the 60's as a novelty product, all they need is a cam on to switch contacts, and the main toggle switch. The toggle switch makes it run, then once moving the camswitch keeps the power going to the motor until a full cam cycle is done.

Cool project anyway!

 

[alec_t]

Er...., R1 and R4 in the sim circuit are shown as 1 milliohm, but I guess you used 1 Megohm on the breadboard

 

[alec_t]

Er...., R1 and R4 in the sim circuit are shown as 1 milliohm, but I guess you used 1 Megohm on the breadboard

Rather than use a bipolar transistor, if you could source a suitable logic-level MOSFET you should be able to drive that ok with the TLC555s running from 3V.

 

[planedoctor]

Er...., R1 and R4 in the sim circuit are shown as 1 milliohm, but I guess you used 1 Megohm on the breadboard

Rather than use a bipolar transistor, if you could source a suitable logic-level MOSFET you should be able to drive that ok with the TLC555s running from 3V.

The R1 and R4 are typo error. They are indeed a 1 Mega-ohm.

I have an IRF510 MOSFET available but it also requires about 5v to switch.

So should I stick with the dual power or source a MOSFET? I honestly don't know where to start looking.
What category do I look for ?

 

[alec_t]

The IRF510 is not a logic level type.

What category do I look for ?

It must be logic-level (so 3V will turn it fully on). It should have a low (tens of milliohms max) RdsOn.

 

[planedoctor]

What about this?

https://www.fairchildsemi.com/ds/RF/RFD3055LE.pdf

Mouser only has +87,000 in stock!

Mouser search-

https://www.mouser.com/Semiconductors/Transistors/MOSFET/_/N-96mrk?P=1z0y3zrZ1z0z63x&Keyword=logic+level+mosfet&FS=True

 

[alec_t]

That FET has too high a gate threshold voltage. Its spec is only 250uA at 3V. Keep hunting.

 

[planedoctor]

So what #'s do I need to look for?

gate threshold voltage = ?
This one has 1mA
https://www.mouser.com/ds/0/NXP/PHP79NQ08LT-53450.pdf



logic-level (so 3V will turn it fully on). Got that one understood

It should have a low (tens of milliohms max) RdsOn. = So under. 10mΩ or .10Ω ???


Sorry, I really have no clue about the FET as to what I need.

 

[alec_t]

So what #'s do I need to look for?

Current rating should be at least 2A (most are). Vgs threshold should be as low as possible, as should Rdson. Ideally the spec would show Rdson at Vgs = 3V test voltage, but the lowest test voltage I found in the specs was 4.5V.
I searched Farnell's site using Vgs threshold maximum = 1V as a search parameter.
Perhaps a IRLML6344TRPBF ? It's rated 30V, 5A, with Rdson = 22milliohms at Vgs = 4.5V. The gate threshold voltage is 0.8. It's a SOT23 package, so you'd need nimble fingers to solder it.

 

[planedoctor]

Thanks so much.