I am trying to repair a broken toy car (a Bee-bot to be exact).

Each of it's two motors controlled by H bridges. One of the motors runs correctly both forward and reverse. The other motor runs only forward.

I believe the schematic is as such (for one motor):

enter image description here

I do not know what Y6, Y5 and 1G is but I assume they are transistors.

Since it has two identical bridges (one for each motor), with one of them working, I can easily compare them.

The signals are confirmed to be accurate low or high, and the motor is running fine in both direction with powered by another source.

When input is applied to input 1 on the GOOD bridge (motor runs fine), I get these voltages:

  • MR+: 0.3V
  • MR-: 3.3V (this is a bit counter-intuitive to me - feels like it should be the opposite? have I measured wrong somehow)?
  • Between Y5 (near MR-) and 1G: ~0.5V

When input is applied to input 1 on the BAD bridge (motor does not run), I get these voltages:

  • MR+: 3.3V
  • MR-: 3.3V
  • Between Y5 (near MR-) and 1G: ~3.2V

Testing the Y5 near MR- as diode with multimeter between MR- and 1G reads 0.06V in both directions - i.e. it looks bad. Testing the other three Y5's on same pins gives expected readings of ~0.6-07V in one direction, UL in other.


  • Is it likely that the Y5 transistor is the single failed point, or should something else be checked as well before replacing it?

  • What can I replace it with? I'm not able to find any specs of "Y5". I assume it is a silicon (~0.6-0.7V drop) NPN transistor, with Base against 1G, Collector against M-, Emitter against Ground. Is this accurate? What else has to match on the replacement? The current component is an SMD but I would be happy to replace it with something larger for easier soldering... Could it be replaced, for example, with a C547B that I may have lying around? :)

  • \$\begingroup\$ Are there any markings on the devices? \$\endgroup\$
    – Trevor_G
    Commented Mar 30, 2017 at 22:05
  • \$\begingroup\$ The only markings are those in the schematics: Y6, Y5, 1G, 470, 102, MR+, MR- \$\endgroup\$ Commented Mar 30, 2017 at 22:08
  • \$\begingroup\$ The resistors with 102 are 1k ohm resistors. The resistors with 470 are 47 ohm resistors. \$\endgroup\$
    – sa_leinad
    Commented May 14, 2019 at 6:55

1 Answer 1


1G are smaller NPN transistors. Generic type:2N2222
Y5 are larger, heavier current NPN transistors to take motor current. MJE181 current rating of 3A might be adequate, a lower voltage rating is OK.
Y6 are large heavier current PNP transistors to take motor current. MJE171 current rating of 3A might be adequate, a lower voltage rating is OK.
With only 3V supply, a low saturation voltage would be beneficial. You'll have to determine transistor base pinout.

Input 1, input 2 are logic compatible. Caution: a logic "high" on both will turn on all transistors, and kill them (or your battery). Only allow one of these inputs to go high. When both are low, the motor should be off.

  • \$\begingroup\$ Thank you! I assumed the base pin on Y5 and Y6 would be connected to the 1Gs, and the base pins on 1G to be connected to the logical signal. Is this not correct? As far as I've tested, only one logic input is ever high at the same time. Though I am worried the broken Y5 (or anything else) could have created a similar scenario, since it seems at least 3 of the 4 transistors are turned on when input 1 is high. \$\endgroup\$ Commented Mar 30, 2017 at 22:54
  • \$\begingroup\$ Thank you for the added details about suitable replacements! \$\endgroup\$ Commented Mar 30, 2017 at 22:56
  • \$\begingroup\$ @JanErikGunnar Yes, 1G's collector feeds a base of Y6 through a 470 ohm resistor. 1G's emitter feeds a base of Y5 directly. \$\endgroup\$
    – glen_geek
    Commented Mar 30, 2017 at 23:04
  • \$\begingroup\$ Maybe I need to clarify that the current components are extremely small, I believe they are SOT-23 packages. I am worried the suggested replacements have too high saturation voltages or have too high threshold voltage on the base? In the current functioning curcuit, the voltage at Y5 base is around 0.5V, if I measured correctly. \$\endgroup\$ Commented Mar 30, 2017 at 23:28
  • \$\begingroup\$ @JanErikGunnar All bipolar NPNs, PNPs have threshold voltage around 0.65v. Search your favourite supplier for replacements Vce>15v, Icmax>1A Hfe>150. (These are for Y5, Y6). \$\endgroup\$
    – glen_geek
    Commented Mar 31, 2017 at 0:38

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